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.

1 files changed, 12 insertions, 2 deletions

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));
 	};