Date and time bugs survive development because everything looks fine in one timezone. I want tests for the edges. Find the date-sensitive logic in my app: trial periods, billing cycles, subscription renewals, scheduling features, 'X days ago' displays, date range filters, expirations of any kind. List what you find. Then write tests covering the known failure modes: 1. Timezone boundaries: a user in a different timezone than the server sees dates on the correct day (a trial ending 'March 15' doesn't end on March 14 for them). 2. Daylight saving transitions: durations that span a DST change are still correct (a 30-day trial is 30 days, not 29 days 23 hours). 3. Month and year boundaries: monthly billing or reports behave correctly on the 31st, in February, and across New Year's. 4. Expiration comparisons: 'is this expired' checks compare in the same timezone on both sides, and expire at the intended instant (start of day vs end of day matters). 5. Storage: dates are stored in a consistent, unambiguous format (UTC or with explicit offsets), not server-local time. If they're stored in local time, flag it as a finding. Use controlled clocks or injected 'now' values so these tests are deterministic. A test that only fails on March 31st is worse than no test. When finished, run the suite and report which date behaviors are verified and where you found actual bugs.