First commit, done on M6 from the PLAN.MD

web/js/engine/time.js

@@ -0,0 +1,31 @@
+// Real-time clock accumulator and in-game calendar helpers.
+
+export function createClock() {
+  return { acc: 0 };
+}
+
+// Feed it the real elapsed ms; it returns how many in-game days are due.
+// dt is capped so a background tab doesn't fast-forward the game on return, and
+// floored at 0: the first animation frame can carry a timestamp slightly before
+// the one the caller recorded, and a negative step would pre-load a whole day.
+export function consumeClock(clock, dtMs, msPerDay, speed) {
+  clock.acc += Math.max(0, Math.min(dtMs, 1000)) * speed;
+  const days = Math.floor(clock.acc / msPerDay);
+  clock.acc -= days * msPerDay;
+  return days;
+}
+
+export function dateOfDay(startDate, day) {
+  const [y, m, d] = startDate.split("-").map(Number);
+  const date = new Date(Date.UTC(y, m - 1, d + day));
+  return {
+    year: date.getUTCFullYear(),
+    month: date.getUTCMonth() + 1,
+    day: date.getUTCDate(),
+    weekday: date.getUTCDay(), // 0 = Sunday
+    iso: date.toISOString().slice(0, 10),
+  };
+}
+
+export const weekOfDay = (day, bal) => Math.floor(day / bal.time.daysPerWeek);
+export const quarterOfDay = (day, bal) => Math.floor(day / bal.time.daysPerQuarter);