feat: add date-key module with local-time formatting

Switches task keys to ISO YYYY-MM-DD. They sort, so finding every day
before today is a string compare instead of re-parsing each key.

Two things worth knowing. fromKey builds from date parts rather than
parsing a bare ISO string, which gets read as UTC and lands on the wrong
day west of Greenwich. addDays goes through setDate, so it stays on the
same calendar day across a DST change instead of adding a flat 24 hours.

src/lib/dates.js

@@ -0,0 +1,44 @@
+const KEY_PATTERN = /^\d{4}-\d{2}-\d{2}$/
+
+/** A `Date` as a local-time `YYYY-MM-DD` key. Sortable, which is what the
+ *  rollover logic relies on to find every day earlier than today. */
+export function toKey(date) {
+  const year = date.getFullYear()
+  const month = String(date.getMonth() + 1).padStart(2, '0')
+  const day = String(date.getDate()).padStart(2, '0')
+  return `${year}-${month}-${day}`
+}
+
+/** Local midnight for a key. Built from parts rather than `new Date(key)`,
+ *  which parses bare ISO dates as UTC and shifts the day west of Greenwich. */
+export function fromKey(key) {
+  const [year, month, day] = key.split('-').map(Number)
+  return new Date(year, month - 1, day)
+}
+
+/** Goes through `setDate`, so it stays on the same calendar day across a DST
+ *  boundary rather than adding a fixed 24 hours. */
+export function addDays(date, amount) {
+  const next = new Date(date.getTime())
+  next.setDate(next.getDate() + amount)
+  return next
+}
+
+export function isKey(value) {
+  return KEY_PATTERN.test(value)
+}
+
+export function labelFor(dateKey, todayKey) {
+  if (dateKey === todayKey) return 'Today'
+  if (dateKey === toKey(addDays(fromKey(todayKey), 1))) return 'Tomorrow'
+  return fromKey(dateKey).toLocaleDateString('en-US', { month: 'short', day: 'numeric' })
+}
+
+export function formatLong(dateKey) {
+  return fromKey(dateKey).toLocaleDateString('en-US', {
+    weekday: 'long',
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })
+}