diff --git a/src/app/page.test.tsx b/src/app/page.test.tsx
index ae266a1..ffe9de7 100644
--- a/src/app/page.test.tsx
+++ b/src/app/page.test.tsx
@@ -500,11 +500,16 @@ describe("Dashboard", () => {
 
   describe("error handling", () => {
     it("shows error message when /api/today fails", async () => {
-      mockFetch.mockResolvedValueOnce({
-        ok: false,
-        status: 500,
-        json: () => Promise.resolve({ error: "Internal server error" }),
-      });
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 500,
+          json: () => Promise.resolve({ error: "Internal server error" }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockUserResponse),
+        });
 
       render(<Dashboard />);
 
@@ -515,20 +520,31 @@ describe("Dashboard", () => {
     });
 
     it("shows setup message when user has no lastPeriodDate", async () => {
-      mockFetch.mockResolvedValueOnce({
-        ok: false,
-        status: 400,
-        json: () =>
-          Promise.resolve({
-            error:
-              "User has no lastPeriodDate set. Please log your period start date first.",
-          }),
-      });
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 400,
+          json: () =>
+            Promise.resolve({
+              error:
+                "User has no lastPeriodDate set. Please log your period start date first.",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: null,
+            }),
+        });
 
       render(<Dashboard />);
 
       await waitFor(() => {
-        expect(screen.getByRole("alert")).toBeInTheDocument();
+        // Multiple alerts may be present (error alert + onboarding banner)
+        const alerts = screen.getAllByRole("alert");
+        expect(alerts.length).toBeGreaterThan(0);
         // Check for the specific help text about getting started
         expect(
           screen.getByText(/please log your period start date to get started/i),
@@ -723,4 +739,295 @@ describe("Dashboard", () => {
       });
     });
   });
+
+  describe("period date modal flow", () => {
+    it("shows onboarding banner when todayData fails but userData shows no lastPeriodDate", async () => {
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 400,
+          json: () =>
+            Promise.resolve({
+              error:
+                "User has no lastPeriodDate set. Please log your period start date first.",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: null,
+            }),
+        });
+
+      render(<Dashboard />);
+
+      await waitFor(() => {
+        expect(
+          screen.getByText(/Set your last period date for accurate tracking/i),
+        ).toBeInTheDocument();
+        expect(
+          screen.getByRole("button", { name: /set date/i }),
+        ).toBeInTheDocument();
+      });
+    });
+
+    it("opens period date modal when clicking Set date button", async () => {
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 400,
+          json: () =>
+            Promise.resolve({
+              error:
+                "User has no lastPeriodDate set. Please log your period start date first.",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: null,
+            }),
+        });
+
+      render(<Dashboard />);
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("button", { name: /set date/i }),
+        ).toBeInTheDocument();
+      });
+
+      fireEvent.click(screen.getByRole("button", { name: /set date/i }));
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("dialog", { name: /set period date/i }),
+        ).toBeInTheDocument();
+      });
+    });
+
+    it("calls POST /api/cycle/period when submitting date in modal", async () => {
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 400,
+          json: () =>
+            Promise.resolve({
+              error:
+                "User has no lastPeriodDate set. Please log your period start date first.",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: null,
+            }),
+        });
+
+      render(<Dashboard />);
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("button", { name: /set date/i }),
+        ).toBeInTheDocument();
+      });
+
+      fireEvent.click(screen.getByRole("button", { name: /set date/i }));
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("dialog", { name: /set period date/i }),
+        ).toBeInTheDocument();
+      });
+
+      // Set up mock for the period POST and subsequent refetch
+      mockFetch.mockClear();
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              message: "Period start date logged successfully",
+              lastPeriodDate: "2024-01-15",
+              cycleDay: 1,
+              phase: "MENSTRUAL",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockTodayResponse),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: "2024-01-15",
+            }),
+        });
+
+      const dateInput = screen.getByLabelText(
+        /when did your last period start/i,
+      );
+      fireEvent.change(dateInput, { target: { value: "2024-01-15" } });
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      await waitFor(() => {
+        expect(mockFetch).toHaveBeenCalledWith("/api/cycle/period", {
+          method: "POST",
+          headers: { "Content-Type": "application/json" },
+          body: JSON.stringify({ startDate: "2024-01-15" }),
+        });
+      });
+    });
+
+    it("closes modal and refetches data after successful submission", async () => {
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 400,
+          json: () =>
+            Promise.resolve({
+              error:
+                "User has no lastPeriodDate set. Please log your period start date first.",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: null,
+            }),
+        });
+
+      render(<Dashboard />);
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("button", { name: /set date/i }),
+        ).toBeInTheDocument();
+      });
+
+      fireEvent.click(screen.getByRole("button", { name: /set date/i }));
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("dialog", { name: /set period date/i }),
+        ).toBeInTheDocument();
+      });
+
+      // Set up mock for the period POST and successful refetch
+      mockFetch.mockClear();
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              message: "Period start date logged successfully",
+              lastPeriodDate: "2024-01-15",
+              cycleDay: 1,
+              phase: "MENSTRUAL",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () => Promise.resolve(mockTodayResponse),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: "2024-01-15",
+            }),
+        });
+
+      const dateInput = screen.getByLabelText(
+        /when did your last period start/i,
+      );
+      fireEvent.change(dateInput, { target: { value: "2024-01-15" } });
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      // Modal should close and dashboard should show normal content
+      await waitFor(() => {
+        expect(
+          screen.queryByRole("dialog", { name: /set period date/i }),
+        ).not.toBeInTheDocument();
+        // Dashboard should now show the decision card
+        expect(screen.getByText("TRAIN")).toBeInTheDocument();
+      });
+    });
+
+    it("shows error in modal when API call fails", async () => {
+      mockFetch
+        .mockResolvedValueOnce({
+          ok: false,
+          status: 400,
+          json: () =>
+            Promise.resolve({
+              error:
+                "User has no lastPeriodDate set. Please log your period start date first.",
+            }),
+        })
+        .mockResolvedValueOnce({
+          ok: true,
+          json: () =>
+            Promise.resolve({
+              ...mockUserResponse,
+              lastPeriodDate: null,
+            }),
+        });
+
+      render(<Dashboard />);
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("button", { name: /set date/i }),
+        ).toBeInTheDocument();
+      });
+
+      fireEvent.click(screen.getByRole("button", { name: /set date/i }));
+
+      await waitFor(() => {
+        expect(
+          screen.getByRole("dialog", { name: /set period date/i }),
+        ).toBeInTheDocument();
+      });
+
+      // Set up mock for failed API call
+      mockFetch.mockClear();
+      mockFetch.mockResolvedValueOnce({
+        ok: false,
+        status: 500,
+        json: () => Promise.resolve({ error: "Failed to update period date" }),
+      });
+
+      const dateInput = screen.getByLabelText(
+        /when did your last period start/i,
+      );
+      fireEvent.change(dateInput, { target: { value: "2024-01-15" } });
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      // Error should appear in modal (there may be multiple alerts - dashboard error + modal error)
+      await waitFor(() => {
+        const alerts = screen.getAllByRole("alert");
+        const modalError = alerts.find((alert) =>
+          alert.textContent?.includes("Failed to update period date"),
+        );
+        expect(modalError).toBeInTheDocument();
+      });
+
+      // Modal should still be open
+      expect(
+        screen.getByRole("dialog", { name: /set period date/i }),
+      ).toBeInTheDocument();
+    });
+  });
 });
diff --git a/src/app/page.tsx b/src/app/page.tsx
index e853630..84725ea 100644
--- a/src/app/page.tsx
+++ b/src/app/page.tsx
@@ -10,6 +10,7 @@ import { MiniCalendar } from "@/components/dashboard/mini-calendar";
 import { NutritionPanel } from "@/components/dashboard/nutrition-panel";
 import { OnboardingBanner } from "@/components/dashboard/onboarding-banner";
 import { OverrideToggles } from "@/components/dashboard/override-toggles";
+import { PeriodDateModal } from "@/components/dashboard/period-date-modal";
 import { DashboardSkeleton } from "@/components/dashboard/skeletons";
 import type {
   CyclePhase,
@@ -54,6 +55,7 @@ export default function Dashboard() {
   const [error, setError] = useState<string | null>(null);
   const [calendarYear, setCalendarYear] = useState(new Date().getFullYear());
   const [calendarMonth, setCalendarMonth] = useState(new Date().getMonth());
+  const [showPeriodModal, setShowPeriodModal] = useState(false);
 
   const handleCalendarMonthChange = useCallback(
     (year: number, month: number) => {
@@ -87,27 +89,62 @@ export default function Dashboard() {
 
   useEffect(() => {
     async function loadData() {
-      try {
-        setLoading(true);
-        setError(null);
+      setLoading(true);
+      setError(null);
 
-        const [today, user] = await Promise.all([
-          fetchTodayData(),
-          fetchUserData(),
-        ]);
+      // Fetch userData and todayData independently so we can show the
+      // onboarding banner even if todayData fails due to missing lastPeriodDate
+      const [todayResult, userResult] = await Promise.allSettled([
+        fetchTodayData(),
+        fetchUserData(),
+      ]);
 
-        setTodayData(today);
-        setUserData(user);
-      } catch (err) {
-        setError(err instanceof Error ? err.message : "An error occurred");
-      } finally {
-        setLoading(false);
+      if (userResult.status === "fulfilled") {
+        setUserData(userResult.value);
       }
+
+      if (todayResult.status === "fulfilled") {
+        setTodayData(todayResult.value);
+      } else {
+        setError(
+          todayResult.reason instanceof Error
+            ? todayResult.reason.message
+            : "Failed to fetch today data",
+        );
+      }
+
+      setLoading(false);
     }
 
     loadData();
   }, [fetchTodayData, fetchUserData]);
 
+  const handlePeriodDateSubmit = async (date: string) => {
+    const response = await fetch("/api/cycle/period", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ startDate: date }),
+    });
+
+    const data = await response.json();
+
+    if (!response.ok) {
+      throw new Error(data.error || "Failed to update period date");
+    }
+
+    // Close modal and refetch all data
+    setShowPeriodModal(false);
+    setError(null);
+
+    const [today, user] = await Promise.all([
+      fetchTodayData(),
+      fetchUserData(),
+    ]);
+
+    setTodayData(today);
+    setUserData(user);
+  };
+
   const handleOverrideToggle = async (override: OverrideType) => {
     if (!userData) return;
 
@@ -160,12 +197,25 @@ export default function Dashboard() {
         {loading && <DashboardSkeleton />}
 
         {error && (
-          <div role="alert" className="text-center py-12">
-            <p className="text-red-500">Error: {error}</p>
-            {error.includes("lastPeriodDate") && (
-              <p className="text-sm text-zinc-400 mt-2">
-                Please log your period start date to get started.
-              </p>
+          <div className="space-y-6">
+            <div role="alert" className="text-center py-12">
+              <p className="text-red-500">Error: {error}</p>
+              {error.includes("lastPeriodDate") && (
+                <p className="text-sm text-zinc-400 mt-2">
+                  Please log your period start date to get started.
+                </p>
+              )}
+            </div>
+            {/* Show onboarding banner even in error state if userData shows missing period date */}
+            {userData && !userData.lastPeriodDate && (
+              <OnboardingBanner
+                status={{
+                  garminConnected: userData.garminConnected,
+                  lastPeriodDate: userData.lastPeriodDate,
+                  notificationTime: userData.notificationTime,
+                }}
+                onSetPeriodDate={() => setShowPeriodModal(true)}
+              />
             )}
           </div>
         )}
@@ -179,6 +229,7 @@ export default function Dashboard() {
                 lastPeriodDate: userData.lastPeriodDate,
                 notificationTime: userData.notificationTime,
               }}
+              onSetPeriodDate={() => setShowPeriodModal(true)}
             />
 
             {/* Cycle Info */}
@@ -231,6 +282,12 @@ export default function Dashboard() {
           </div>
         )}
       </main>
+
+      <PeriodDateModal
+        isOpen={showPeriodModal}
+        onClose={() => setShowPeriodModal(false)}
+        onSubmit={handlePeriodDateSubmit}
+      />
     </div>
   );
 }
diff --git a/src/components/dashboard/period-date-modal.test.tsx b/src/components/dashboard/period-date-modal.test.tsx
new file mode 100644
index 0000000..d0bfcfc
--- /dev/null
+++ b/src/components/dashboard/period-date-modal.test.tsx
@@ -0,0 +1,215 @@
+// ABOUTME: Tests for PeriodDateModal component that allows users to set their last period date.
+// ABOUTME: Tests modal visibility, date validation, form submission, and accessibility.
+
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { describe, expect, it, vi } from "vitest";
+import { PeriodDateModal } from "./period-date-modal";
+
+describe("PeriodDateModal", () => {
+  const defaultProps = {
+    isOpen: true,
+    onClose: vi.fn(),
+    onSubmit: vi.fn(),
+  };
+
+  describe("visibility", () => {
+    it("renders when isOpen is true", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      expect(
+        screen.getByRole("dialog", { name: /set period date/i }),
+      ).toBeInTheDocument();
+    });
+
+    it("does not render when isOpen is false", () => {
+      render(<PeriodDateModal {...defaultProps} isOpen={false} />);
+      expect(
+        screen.queryByRole("dialog", { name: /set period date/i }),
+      ).not.toBeInTheDocument();
+    });
+  });
+
+  describe("form elements", () => {
+    it("renders a date input", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      expect(
+        screen.getByLabelText(/when did your last period start/i),
+      ).toBeInTheDocument();
+    });
+
+    it("renders a cancel button", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      expect(
+        screen.getByRole("button", { name: /cancel/i }),
+      ).toBeInTheDocument();
+    });
+
+    it("renders a submit button", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      expect(screen.getByRole("button", { name: /save/i })).toBeInTheDocument();
+    });
+
+    it("sets max date to today to prevent future dates", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      const input = screen.getByLabelText(/when did your last period start/i);
+      const today = new Date().toISOString().split("T")[0];
+      expect(input).toHaveAttribute("max", today);
+    });
+  });
+
+  describe("closing behavior", () => {
+    it("calls onClose when cancel button is clicked", () => {
+      const onClose = vi.fn();
+      render(<PeriodDateModal {...defaultProps} onClose={onClose} />);
+
+      fireEvent.click(screen.getByRole("button", { name: /cancel/i }));
+      expect(onClose).toHaveBeenCalledTimes(1);
+    });
+
+    it("calls onClose when backdrop is clicked", () => {
+      const onClose = vi.fn();
+      render(<PeriodDateModal {...defaultProps} onClose={onClose} />);
+
+      const backdrop = screen.getByTestId("modal-backdrop");
+      fireEvent.click(backdrop);
+      expect(onClose).toHaveBeenCalledTimes(1);
+    });
+
+    it("calls onClose when ESC key is pressed", () => {
+      const onClose = vi.fn();
+      render(<PeriodDateModal {...defaultProps} onClose={onClose} />);
+
+      fireEvent.keyDown(document, { key: "Escape" });
+      expect(onClose).toHaveBeenCalledTimes(1);
+    });
+
+    it("does not close when clicking inside the modal content", () => {
+      const onClose = vi.fn();
+      render(<PeriodDateModal {...defaultProps} onClose={onClose} />);
+
+      const dialog = screen.getByRole("dialog");
+      fireEvent.click(dialog);
+      expect(onClose).not.toHaveBeenCalled();
+    });
+  });
+
+  describe("form submission", () => {
+    it("calls onSubmit with the selected date when form is submitted", async () => {
+      const onSubmit = vi.fn().mockResolvedValue(undefined);
+      render(<PeriodDateModal {...defaultProps} onSubmit={onSubmit} />);
+
+      const input = screen.getByLabelText(/when did your last period start/i);
+      fireEvent.change(input, { target: { value: "2024-01-15" } });
+
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      await waitFor(() => {
+        expect(onSubmit).toHaveBeenCalledWith("2024-01-15");
+      });
+    });
+
+    it("does not submit when no date is selected", async () => {
+      const onSubmit = vi.fn();
+      render(<PeriodDateModal {...defaultProps} onSubmit={onSubmit} />);
+
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      // Should show validation error, not call onSubmit
+      await waitFor(() => {
+        expect(onSubmit).not.toHaveBeenCalled();
+      });
+    });
+
+    it("shows loading state during submission", async () => {
+      const onSubmit = vi
+        .fn()
+        .mockImplementation(
+          () => new Promise((resolve) => setTimeout(resolve, 100)),
+        );
+      render(<PeriodDateModal {...defaultProps} onSubmit={onSubmit} />);
+
+      const input = screen.getByLabelText(/when did your last period start/i);
+      fireEvent.change(input, { target: { value: "2024-01-15" } });
+
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      expect(
+        screen.getByRole("button", { name: /saving/i }),
+      ).toBeInTheDocument();
+      expect(screen.getByRole("button", { name: /saving/i })).toBeDisabled();
+    });
+
+    it("disables cancel button during submission", async () => {
+      const onSubmit = vi
+        .fn()
+        .mockImplementation(
+          () => new Promise((resolve) => setTimeout(resolve, 100)),
+        );
+      render(<PeriodDateModal {...defaultProps} onSubmit={onSubmit} />);
+
+      const input = screen.getByLabelText(/when did your last period start/i);
+      fireEvent.change(input, { target: { value: "2024-01-15" } });
+
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      expect(screen.getByRole("button", { name: /cancel/i })).toBeDisabled();
+    });
+  });
+
+  describe("error handling", () => {
+    it("displays error message when onSubmit throws", async () => {
+      const onSubmit = vi.fn().mockRejectedValue(new Error("API failed"));
+      render(<PeriodDateModal {...defaultProps} onSubmit={onSubmit} />);
+
+      const input = screen.getByLabelText(/when did your last period start/i);
+      fireEvent.change(input, { target: { value: "2024-01-15" } });
+
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      await waitFor(() => {
+        expect(screen.getByRole("alert")).toBeInTheDocument();
+        expect(screen.getByText(/api failed/i)).toBeInTheDocument();
+      });
+    });
+
+    it("clears error when date is changed", async () => {
+      const onSubmit = vi.fn().mockRejectedValue(new Error("API failed"));
+      render(<PeriodDateModal {...defaultProps} onSubmit={onSubmit} />);
+
+      const input = screen.getByLabelText(/when did your last period start/i);
+      fireEvent.change(input, { target: { value: "2024-01-15" } });
+      fireEvent.click(screen.getByRole("button", { name: /save/i }));
+
+      await waitFor(() => {
+        expect(screen.getByRole("alert")).toBeInTheDocument();
+      });
+
+      // Change the date
+      fireEvent.change(input, { target: { value: "2024-01-16" } });
+
+      expect(screen.queryByRole("alert")).not.toBeInTheDocument();
+    });
+  });
+
+  describe("accessibility", () => {
+    it("has proper dialog role and aria-label", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      const dialog = screen.getByRole("dialog");
+      expect(dialog).toHaveAttribute("aria-labelledby");
+    });
+
+    it("focuses the date input when modal opens", async () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      const input = screen.getByLabelText(/when did your last period start/i);
+
+      await waitFor(() => {
+        expect(document.activeElement).toBe(input);
+      });
+    });
+
+    it("has aria-modal attribute", () => {
+      render(<PeriodDateModal {...defaultProps} />);
+      const dialog = screen.getByRole("dialog");
+      expect(dialog).toHaveAttribute("aria-modal", "true");
+    });
+  });
+});
diff --git a/src/components/dashboard/period-date-modal.tsx b/src/components/dashboard/period-date-modal.tsx
new file mode 100644
index 0000000..52cd21b
--- /dev/null
+++ b/src/components/dashboard/period-date-modal.tsx
@@ -0,0 +1,158 @@
+// ABOUTME: Modal component for setting the last period date.
+// ABOUTME: Used during onboarding when users need to initialize their cycle tracking.
+
+"use client";
+
+import { useCallback, useEffect, useRef, useState } from "react";
+
+interface PeriodDateModalProps {
+  isOpen: boolean;
+  onClose: () => void;
+  onSubmit: (date: string) => Promise<void>;
+}
+
+export function PeriodDateModal({
+  isOpen,
+  onClose,
+  onSubmit,
+}: PeriodDateModalProps) {
+  const [selectedDate, setSelectedDate] = useState("");
+  const [error, setError] = useState<string | null>(null);
+  const [isSubmitting, setIsSubmitting] = useState(false);
+  const inputRef = useRef<HTMLInputElement>(null);
+
+  const today = new Date().toISOString().split("T")[0];
+
+  // Focus input when modal opens
+  useEffect(() => {
+    if (isOpen && inputRef.current) {
+      inputRef.current.focus();
+    }
+  }, [isOpen]);
+
+  // Handle ESC key
+  useEffect(() => {
+    if (!isOpen) return;
+
+    const handleKeyDown = (e: KeyboardEvent) => {
+      if (e.key === "Escape" && !isSubmitting) {
+        onClose();
+      }
+    };
+
+    document.addEventListener("keydown", handleKeyDown);
+    return () => document.removeEventListener("keydown", handleKeyDown);
+  }, [isOpen, isSubmitting, onClose]);
+
+  // Clear error when date changes
+  const handleDateChange = useCallback(
+    (e: React.ChangeEvent<HTMLInputElement>) => {
+      setSelectedDate(e.target.value);
+      setError(null);
+    },
+    [],
+  );
+
+  const handleSubmit = async (e: React.FormEvent) => {
+    e.preventDefault();
+
+    if (!selectedDate) {
+      return;
+    }
+
+    setIsSubmitting(true);
+    setError(null);
+
+    try {
+      await onSubmit(selectedDate);
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Failed to save date");
+    } finally {
+      setIsSubmitting(false);
+    }
+  };
+
+  const handleBackdropClick = (e: React.MouseEvent) => {
+    if (e.target === e.currentTarget && !isSubmitting) {
+      onClose();
+    }
+  };
+
+  if (!isOpen) {
+    return null;
+  }
+
+  return (
+    // biome-ignore lint/a11y/useKeyWithClickEvents: Keyboard navigation handled by ESC key listener
+    // biome-ignore lint/a11y/noStaticElementInteractions: Backdrop click-to-close is a convenience feature
+    <div
+      data-testid="modal-backdrop"
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/50"
+      onClick={handleBackdropClick}
+    >
+      {/* biome-ignore lint/a11y/useKeyWithClickEvents: Click handler prevents event bubbling, not user interaction */}
+      <div
+        role="dialog"
+        aria-modal="true"
+        aria-labelledby="period-modal-title"
+        className="bg-white dark:bg-zinc-800 rounded-lg shadow-xl p-6 w-full max-w-md mx-4"
+        onClick={(e) => e.stopPropagation()}
+      >
+        <h2
+          id="period-modal-title"
+          className="text-lg font-semibold mb-4 text-gray-900 dark:text-gray-100"
+        >
+          Set Period Date
+        </h2>
+
+        <form onSubmit={handleSubmit}>
+          <div className="mb-4">
+            <label
+              htmlFor="period-date"
+              className="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2"
+            >
+              When did your last period start?
+            </label>
+            <input
+              ref={inputRef}
+              type="date"
+              id="period-date"
+              value={selectedDate}
+              onChange={handleDateChange}
+              max={today}
+              disabled={isSubmitting}
+              className="w-full px-3 py-2 border border-gray-300 dark:border-zinc-600 rounded-md shadow-sm focus:outline-none focus:ring-2 focus:ring-purple-500 focus:border-purple-500 dark:bg-zinc-700 dark:text-gray-100 disabled:opacity-50"
+            />
+          </div>
+
+          {error && (
+            <div
+              role="alert"
+              className="mb-4 p-3 text-sm text-red-700 bg-red-100 rounded-md dark:text-red-300 dark:bg-red-900/30"
+            >
+              {error}
+            </div>
+          )}
+
+          <div className="flex gap-3 justify-end">
+            <button
+              type="button"
+              onClick={onClose}
+              disabled={isSubmitting}
+              className="px-4 py-2 text-sm font-medium text-gray-700 dark:text-gray-300 bg-gray-100 dark:bg-zinc-700 rounded-md hover:bg-gray-200 dark:hover:bg-zinc-600 disabled:opacity-50"
+            >
+              Cancel
+            </button>
+            <button
+              type="submit"
+              disabled={isSubmitting}
+              className="px-4 py-2 text-sm font-medium text-white bg-purple-600 rounded-md hover:bg-purple-700 disabled:opacity-50"
+            >
+              {isSubmitting ? "Saving..." : "Save"}
+            </button>
+          </div>
+        </form>
+      </div>
+    </div>
+  );
+}