5 files changed, 105 insertions, 11 deletions
diff --git a/src/lib/server/services/routine-actions.ts b/src/lib/server/services/routine-actions.ts
index 28a25cc..44ba2b8 100644
--- a/src/lib/server/services/routine-actions.ts
+++ b/src/lib/server/services/routine-actions.ts
@@ -1,6 +1,7 @@
 import { fail, type ActionFailure } from '@sveltejs/kit';
 import { archive, complete, edit, loadCurrent } from './routines';
 import { parseFormString, parseFrequencyDays } from './validation';
+import { instantFromDay } from '$lib/shared/temporal';
 
 /** Shared routine edit/complete actions; rejects missing/archived routines. */
 
@@ -57,12 +58,17 @@ export async function completeRoutineAction(data: FormData, routineId: number) {
 		return fail(400, { error: 'Routine archivée' });
 	}
 
+	const now = Temporal.Now.instant();
+	const day = parseFormString(data.get('doneAt'));
+	const doneAt = day
+		? instantFromDay(day, now)
+		: now.toString({ smallestUnit: 'millisecond' });
+	if (doneAt === null) {
+		return fail(400, { error: 'Date invalide' });
+	}
+
 	try {
-		await complete(
-			routineId,
-			parseFormString(data.get('note')),
-			Temporal.Now.instant().toString({ smallestUnit: 'millisecond' }),
-		);
+		await complete(routineId, parseFormString(data.get('note')), doneAt);
 	} catch {
 		return fail(400, { error: 'Routine introuvable' });
 	}
diff --git a/src/lib/shared/temporal.test.ts b/src/lib/shared/temporal.test.ts
new file mode 100644
index 0000000..ef3477a
--- /dev/null
+++ b/src/lib/shared/temporal.test.ts
@@ -0,0 +1,35 @@
+import { describe, expect, it } from 'vitest';
+import { instantFromDay } from './temporal';
+
+// A weekday well inside the year so noon UTC never collides with a clamp.
+const NOW = Temporal.Instant.from('2026-09-20T08:00:00.000Z');
+
+describe('instantFromDay', () => {
+	it('stamps a past day at noon UTC', () => {
+		expect(instantFromDay('2026-09-13', NOW)).toBe('2026-09-13T12:00:00.000Z');
+	});
+
+	it('clamps today to the current instant', () => {
+		expect(instantFromDay('2026-09-20', NOW)).toBe('2026-09-20T08:00:00.000Z');
+	});
+
+	it('keeps a past day that is only hours old as-is', () => {
+		// 21:00 the day before: noon of that day is still in the past.
+		expect(instantFromDay('2026-09-19', NOW)).toBe('2026-09-19T12:00:00.000Z');
+	});
+
+	it('accepts a day a few hours ahead and clamps it to now', () => {
+		const late = Temporal.Instant.from('2026-09-20T22:00:00.000Z');
+		expect(instantFromDay('2026-09-21', late)).toBe('2026-09-20T22:00:00.000Z');
+	});
+
+	it('rejects a day more than a day ahead', () => {
+		expect(instantFromDay('2026-09-22', NOW)).toBeNull();
+	});
+
+	it('rejects malformed and impossible dates', () => {
+		expect(instantFromDay('', NOW)).toBeNull();
+		expect(instantFromDay('hier', NOW)).toBeNull();
+		expect(instantFromDay('2026-02-30', NOW)).toBeNull();
+	});
+});
diff --git a/src/lib/shared/temporal.ts b/src/lib/shared/temporal.ts
index 103f209..6fd085d 100644
--- a/src/lib/shared/temporal.ts
+++ b/src/lib/shared/temporal.ts
@@ -24,3 +24,29 @@ export function formatDayMonthYear(iso: string): string {
 		year: 'numeric',
 	});
 }
+
+const ONE_DAY_MS = 86_400_000;
+
+/**
+ * Stamp to store for a `YYYY-MM-DD` form day: noon UTC — a midnight stamp can
+ * read back as the wrong day in a timezone ahead of UTC. Clamped to `now` when
+ * the day is still ahead of the clock, and null for a malformed day or one
+ * more than a day in the future (timezones ahead of UTC make "today" fuzzy).
+ */
+export function instantFromDay(
+	day: string,
+	now: Temporal.Instant,
+): string | null {
+	let midnight: Temporal.Instant;
+	try {
+		midnight = Temporal.PlainDate.from(day.trim())
+			.toZonedDateTime('UTC')
+			.toInstant();
+	} catch {
+		return null;
+	}
+	const noon = midnight.add({ hours: 12 });
+	if (noon.epochMilliseconds - now.epochMilliseconds > ONE_DAY_MS) return null;
+	const stamp = Temporal.Instant.compare(noon, now) > 0 ? now : noon;
+	return stamp.toString({ smallestUnit: 'millisecond' });
+}
diff --git a/src/routes/projects/[projectId]/routine/[routineId]/+page.svelte b/src/routes/projects/[projectId]/routine/[routineId]/+page.svelte
index ea292ae..359fd5b 100644
--- a/src/routes/projects/[projectId]/routine/[routineId]/+page.svelte
+++ b/src/routes/projects/[projectId]/routine/[routineId]/+page.svelte
@@ -17,7 +17,18 @@
 	let frequency = $state('');
 	let description = $state('');
 	let note = $state('');
+	let today = $state('');
+	let day = $state('');
 	let saved = $state(false);
+	// Filled after mount: the server's timezone is not the viewer's.
+	$effect(() => {
+		if (today) return;
+		today = Temporal.Now.instant()
+			.toZonedDateTimeISO(Temporal.Now.timeZoneId())
+			.toPlainDate()
+			.toString();
+		day = today;
+	});
 	$effect(() => {
 		name = data.routine.name;
 		frequency = `${data.routine.frequencyDays}d`;
@@ -160,6 +171,7 @@
 			use:enhance={() => {
 				return async ({ update }) => {
 					note = '';
+					day = today;
 					await update({ reset: false });
 				};
 			}}
@@ -172,6 +184,14 @@
 				placeholder="Note (optionnel)"
 				autocomplete="off"
 			/>
+			<input
+				class="form-input done-date"
+				type="date"
+				name="doneAt"
+				bind:value={day}
+				max={today}
+				aria-label="Date de réalisation"
+			/>
 			<Button variant="done" size="lg">Fait</Button>
 		</form>
 	{/if}
@@ -244,10 +264,17 @@
 	.add-completion {
 		flex-direction: row;
 		align-items: center;
+		flex-wrap: wrap;
 	}
 
+	/* Basis wider than the phone width so the note wraps instead of collapsing. */
 	.add-completion .form-input {
-		flex: 1;
+		flex: 1 1 12rem;
+	}
+
+	.add-completion .done-date {
+		flex: 0 0 auto;
+		width: 11rem;
 	}
 
 	.completion-date {
diff --git a/src/temporal-ambient.d.ts b/src/temporal-ambient.d.ts
index 87933b9..c5ec34f 100644
--- a/src/temporal-ambient.d.ts
+++ b/src/temporal-ambient.d.ts
@@ -36,6 +36,7 @@ declare global {
 			add(durationLike: DurationLike): ZonedDateTime;
 			startOfDay(): ZonedDateTime;
 			toInstant(): Instant;
+			toPlainDate(): PlainDate;
 			toLocaleString(
 				locales?: string | string[],
 				options?: Intl.DateTimeFormatOptions,
@@ -47,13 +48,12 @@ declare global {
 			add(durationLike: DurationLike): PlainDate;
 			subtract(durationLike: DurationLike): PlainDate;
 			toZonedDateTime(timeZone: TimeZoneId): ZonedDateTime;
+			toString(): string;
 		}
 		namespace PlainDate {
-			function from(plainDateLike: {
-				year: number;
-				month: number;
-				day: number;
-			}): PlainDate;
+			function from(
+				plainDateLike: { year: number; month: number; day: number } | string,
+			): PlainDate;
 		}
 		namespace Now {
 			function instant(): Instant;