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',
+  })
+}

src/lib/dates.test.js

@@ -0,0 +1,89 @@
+import { describe, it, expect } from 'vitest'
+import { toKey, fromKey, addDays, isKey, labelFor, formatLong } from './dates.js'
+
+describe('toKey', () => {
+  it('formats a date as local YYYY-MM-DD', () => {
+    expect(toKey(new Date(2026, 7, 15))).toBe('2026-08-15')
+  })
+
+  it('zero-pads single-digit months and days', () => {
+    expect(toKey(new Date(2026, 0, 5))).toBe('2026-01-05')
+  })
+
+  it('uses local time, not UTC', () => {
+    // 23:30 local on the 15th must not roll forward to the 16th.
+    expect(toKey(new Date(2026, 7, 15, 23, 30))).toBe('2026-08-15')
+  })
+})
+
+describe('fromKey', () => {
+  it('returns local midnight for the key', () => {
+    const date = fromKey('2026-08-15')
+    expect(date.getFullYear()).toBe(2026)
+    expect(date.getMonth()).toBe(7)
+    expect(date.getDate()).toBe(15)
+    expect(date.getHours()).toBe(0)
+  })
+
+  it('round-trips with toKey', () => {
+    expect(toKey(fromKey('2026-08-15'))).toBe('2026-08-15')
+  })
+})
+
+describe('addDays', () => {
+  it('advances across a month boundary', () => {
+    expect(toKey(addDays(new Date(2026, 7, 31), 1))).toBe('2026-09-01')
+  })
+
+  it('advances across a year boundary', () => {
+    expect(toKey(addDays(new Date(2026, 11, 31), 1))).toBe('2027-01-01')
+  })
+
+  it('does not mutate its argument', () => {
+    const date = new Date(2026, 7, 15)
+    addDays(date, 5)
+    expect(toKey(date)).toBe('2026-08-15')
+  })
+})
+
+describe('isKey', () => {
+  it('accepts ISO date keys', () => {
+    expect(isKey('2026-08-15')).toBe(true)
+  })
+
+  it('rejects legacy toDateString keys', () => {
+    expect(isKey('Sat Aug 15 2026')).toBe(false)
+  })
+
+  it('rejects arbitrary strings', () => {
+    expect(isKey('not-a-date')).toBe(false)
+  })
+})
+
+describe('labelFor', () => {
+  it('labels today', () => {
+    expect(labelFor('2026-08-15', '2026-08-15')).toBe('Today')
+  })
+
+  it('labels tomorrow', () => {
+    expect(labelFor('2026-08-16', '2026-08-15')).toBe('Tomorrow')
+  })
+
+  it('labels tomorrow across a month boundary', () => {
+    expect(labelFor('2026-09-01', '2026-08-31')).toBe('Tomorrow')
+  })
+
+  it('labels tomorrow across a year boundary', () => {
+    expect(labelFor('2027-01-01', '2026-12-31')).toBe('Tomorrow')
+  })
+
+  it('falls back to a short date for other days', () => {
+    expect(labelFor('2026-08-20', '2026-08-15')).toBe('Aug 20')
+  })
+})
+
+describe('formatLong', () => {
+  it('renders the long-form date used in the header', () => {
+    expect(formatLong('2026-08-15')).toBe('Saturday, August 15, 2026')
+  })
+})