feat: duration parser and date utilities

src/__tests__/date.test.ts

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+import { toDateString, todayString, addDays } from '../utils/date';
+
+describe('date utils', () => {
+  it('formats a date as YYYY-MM-DD', () => {
+    expect(toDateString(new Date('2026-05-20T10:00:00'))).toBe('2026-05-20');
+  });
+
+  it('addDays adds days correctly', () => {
+    expect(addDays('2026-05-20', 3)).toBe('2026-05-23');
+  });
+
+  it('addDays handles month boundary', () => {
+    expect(addDays('2026-05-30', 2)).toBe('2026-06-01');
+  });
+});

src/__tests__/duration.test.ts

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+import { parseDuration, formatDuration } from '../utils/duration';
+
+describe('parseDuration', () => {
+  it('parses hours', () => expect(parseDuration('2h')).toBe(120));
+  it('parses hours with space', () => expect(parseDuration('2 h')).toBe(120));
+  it('parses minutes with m', () => expect(parseDuration('90m')).toBe(90));
+  it('parses minutes with min', () => expect(parseDuration('45min')).toBe(45));
+  it('parses plain number as minutes', () => expect(parseDuration('60')).toBe(60));
+  it('parses decimal hours', () => expect(parseDuration('1.5h')).toBe(90));
+  it('returns null for invalid input', () => expect(parseDuration('abc')).toBeNull());
+  it('returns null for zero', () => expect(parseDuration('0m')).toBeNull());
+  it('returns null for empty string', () => expect(parseDuration('')).toBeNull());
+});
+
+describe('formatDuration', () => {
+  it('formats whole hours', () => expect(formatDuration(120)).toBe('2h'));
+  it('formats minutes only', () => expect(formatDuration(45)).toBe('45m'));
+  it('formats hours and minutes', () => expect(formatDuration(90)).toBe('1h 30m'));
+});

src/utils/date.ts

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+export function toDateString(date: Date): string {
+  return date.toISOString().slice(0, 10);
+}
+
+export function todayString(): string {
+  return toDateString(new Date());
+}
+
+export function addDays(dateStr: string, days: number): string {
+  const d = new Date(dateStr + 'T12:00:00');
+  d.setDate(d.getDate() + days);
+  return toDateString(d);
+}

src/utils/duration.ts

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+export function parseDuration(input: string): number | null {
+  const s = input.trim().toLowerCase();
+  if (!s) return null;
+
+  const hoursMatch = s.match(/^(\d+(?:\.\d+)?)\s*h(?:ours?)?$/);
+  if (hoursMatch) {
+    const minutes = Math.round(parseFloat(hoursMatch[1]!) * 60);
+    return minutes > 0 ? minutes : null;
+  }
+
+  const minutesMatch = s.match(/^(\d+)\s*m(?:in(?:utes?)?)?$/);
+  if (minutesMatch) {
+    const minutes = parseInt(minutesMatch[1]!, 10);
+    return minutes > 0 ? minutes : null;
+  }
+
+  const plainMatch = s.match(/^(\d+)$/);
+  if (plainMatch) {
+    const minutes = parseInt(plainMatch[1]!, 10);
+    return minutes > 0 ? minutes : null;
+  }
+
+  return null;
+}
+
+export function formatDuration(minutes: number): string {
+  const h = Math.floor(minutes / 60);
+  const m = minutes % 60;
+  if (h === 0) return `${m}m`;
+  if (m === 0) return `${h}h`;
+  return `${h}h ${m}m`;
+}