Time Converters

Time is the area where plausible-looking code most often turns out to be wrong. These tools cover the conversions involved and, just as usefully, make the common failure modes visible.

Seconds or milliseconds?

Count the digits. A current-era timestamp in seconds has 10 digits; in milliseconds it has 13, and in microseconds 16. Unix tools, Python's time.time() and Go use seconds. JavaScript's Date.now() and Java use milliseconds. Mixing them puts your date either in January 1970 or tens of thousands of years in the future — and a date showing as 1 January 1970 almost always means the value was zero, null or an empty string rather than that something genuinely happened then.

Timestamps have no timezone

A Unix timestamp is an absolute count of seconds since 1970-01-01 00:00:00 UTC, identical everywhere on Earth. A timezone only enters when you format it for display. This is exactly why timestamps are the right thing to store and timezone is a presentation concern.

Offsets are also not all whole hours. India is UTC+05:30, Nepal UTC+05:45, and parts of Australia UTC+08:45. Assuming whole hours is a recurring scheduling bug, as is assuming a location's offset never changes.

The two remaining traps

Daylight saving. Regions change clocks on different dates, so "9am New York time" moves relative to London twice a year. Schedule in UTC where you can, and avoid the hour that shifts — a job set for 02:30 local may be skipped or run twice.

Inclusive versus exclusive dates. From 1 March to 3 March is two nights but three days if both ends count. Contracts and notice periods usually mean the inclusive reading.