From f6687db926e03eb44e5d72d321021f7738600357 Mon Sep 17 00:00:00 2001
From: Martin Kagamino Lehoux <martin@lehoux.net>
Date: Tue, 8 Sep 2026 09:56:55 +0200
Subject: Anchor routine due dates on UTC calendar days

Daily routine done yesterday is due today regardless of time of day;
same anchoring in health series so chart and badge agree.
---
 src/lib/shared/routine-metrics.test.ts | 15 +++++++++++++++
 src/lib/shared/routine-metrics.ts      | 14 ++++++++++++--
 2 files changed, 27 insertions(+), 2 deletions(-)

(limited to 'src/lib/shared')

diff --git a/src/lib/shared/routine-metrics.test.ts b/src/lib/shared/routine-metrics.test.ts
index a63069c..1df39ab 100644
--- a/src/lib/shared/routine-metrics.test.ts
+++ b/src/lib/shared/routine-metrics.test.ts
@@ -63,6 +63,21 @@ describe('computeRoutineState', () => {
 		const state = computeRoutineState(routine, [], now);
 		expect(state.health).toBe(75);
 	});
+	it('is due the calendar day after completion for a daily routine', () => {
+		const daily = { ...routine, frequencyDays: 1 };
+		const doneAt = '2026-09-07T20:00:00.000Z'; // yesterday evening
+		const now = Date.parse('2026-09-08T08:00:00.000Z'); // this morning, <24h later
+		const state = computeRoutineState(daily, [{ routineId: 1, doneAt }], now);
+		expect(state.status).toBe('due');
+	});
+
+	it('is ok later the same day it was done for a daily routine', () => {
+		const daily = { ...routine, frequencyDays: 1 };
+		const doneAt = '2026-09-08T08:00:00.000Z';
+		const now = Date.parse('2026-09-08T20:00:00.000Z');
+		const state = computeRoutineState(daily, [{ routineId: 1, doneAt }], now);
+		expect(state.status).toBe('ok');
+	});
 });
 
 describe('computeHealthSeries', () => {
diff --git a/src/lib/shared/routine-metrics.ts b/src/lib/shared/routine-metrics.ts
index 99e5c56..59413c2 100644
--- a/src/lib/shared/routine-metrics.ts
+++ b/src/lib/shared/routine-metrics.ts
@@ -1,8 +1,16 @@
 // Pure routine metrics, shared by server services and UI. No db access.
 // All timestamps are ISO strings; all math is done in ms then converted to days.
+// Due dates anchor on UTC calendar days, not exact 24h intervals: a daily
+// routine done yesterday is due today, whatever the time of day.
 
 const DAY_MS = 24 * 60 * 60 * 1000;
 
+/** Midnight UTC starting the calendar day containing ms. */
+function startOfUtcDay(ms: number): number {
+	const d = new Date(ms);
+	return Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate());
+}
+
 export type RoutineStatus = 'ok' | 'due' | 'late' | 'veryLate';
 
 export interface RoutineInput {
@@ -48,7 +56,9 @@ export function computeRoutineState(
 	const referenceMs =
 		lastDoneMs ?? (Number.isFinite(createdMs) ? createdMs : now);
 	const periodMs = routine.frequencyDays * DAY_MS;
-	const dueMs = referenceMs + periodMs;
+	// Calendar-day anchor: the period starts at midnight of the reference day,
+	// so any new calendar day past the frequency flips the routine to due.
+	const dueMs = startOfUtcDay(referenceMs) + periodMs;
 	const rawRatio = (now - dueMs) / periodMs;
 	const overdueRatio = Math.max(0, rawRatio);
 	// Buckets on overdue time in units of the routine's own frequency:
@@ -127,7 +137,7 @@ export function computeHealthSeries(
 			refMs = completionMs[nextCompletion];
 			nextCompletion++;
 		}
-		const dueMs = refMs + periodMs;
+		const dueMs = startOfUtcDay(refMs) + periodMs;
 		const rawRatio = (t - dueMs) / periodMs;
 		return Math.max(0, Math.round(100 - Math.max(0, rawRatio) * 100));
 	};
-- 
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