Incorrect Date Handling in Vibecoded Apps

Check if dates display correctly relative to your timezone. Create a record and see if the displayed creation date matches the current date and time in your local timezone. Then change your system timezone and reload — do the dates shift incorrectly? AI-generated apps frequently store local times without timezone information, causing dates to shift when viewed from different timezones.

Look for off-by-one errors at date boundaries. Schedule something for midnight, or select a date range that crosses a month boundary. Test dates around daylight saving time transitions. These edge cases consistently trip up vibecoded date logic.

Change your system timezone to UTC+12 and UTC-12 and compare how dates display. Create records in one timezone and view them in another. Select date ranges in a date picker and verify the start and end dates include the expected days — many vibecoded apps exclude the end date or include an extra day.

Test date inputs with different locale formats. Enter dates as DD/MM/YYYY and MM/DD/YYYY to see if the app interprets them correctly. Check how the app handles February 29 on leap years, December 31 to January 1 transitions, and dates far in the past or future.

Store all dates in UTC on the server and convert to local time only when displaying to the user. Use ISO 8601 format for all date strings in API responses. Avoid JavaScript's native Date constructor for parsing strings — use a library like date-fns or dayjs that handles timezone conversion explicitly.

For date ranges, be explicit about whether boundaries are inclusive or exclusive and document this behavior. Use date-only types (without time) when time is irrelevant to avoid timezone-induced date shifts. Always test with multiple timezones before considering date logic complete.