Time bugs hide in units, zones and calendars. A timestamp in milliseconds read as seconds lands more than 50,000 years from now. A meeting set for 9:00 EST is an hour off all summer. A job scheduled for 2:30 a.m. can run late or not at all on the night the clocks spring forward, and one at 1:30 a.m. can run twice when they fall back.
The Unix timestamp converter reads seconds, milliseconds, microseconds and nanoseconds by their size, as well as ISO 8601 and email-style dates, and shows the instant in any zone. The time zone converter puts one moment in several places at once, with daylight saving and the date shift spelled out, and when you type an abbreviation like EST or BST it says which zone it read. The cron builder explains an expression in plain English and lists its next runs in your zone, the date calculator counts days and business days, and the duration tool reads and writes ISO 8601 durations such as P1DT2H30M.
Two habits prevent most of the trouble. Store and send instants in UTC, and convert to local time only for display. And name zones the IANA way, America/New_York rather than EST, because abbreviations are ambiguous and don’t follow daylight saving: IST alone means India, Israel or Ireland.