Why the time zone is required
A calendar date and time describe a wall clock, not an instant. 12:00 on 4 July 2026 happens at 16:00 UTC in New York, 11:00 UTC in Paris and 06:30 UTC in Kolkata. Many tools quietly use your computer's zone, which gives a different epoch depending on where the code runs. Here the zone is a visible, required field, and the result restates it.
Daylight-saving gaps and overlaps
On 8 March 2026, New York clocks jump from 02:00 to 03:00, so 02:30 never happens that day. On 1 November 2026 they fall back from 02:00 to 01:00, so 01:30 happens twice: first at UTC-4 and again at UTC-5. Converting either time without a policy produces a guess.
When this happens, the converter shows both candidate instants and asks which one you want. "Earlier" and "Later" pick a specific occurrence. "Compatible" matches what JavaScript's Temporal API, java.time and most databases do: in a gap it moves forward by the gap length, and in an overlap it takes the earlier instant.
Questions
- What is the epoch for midnight today?
- Use the Today button in the date fields to set today's date at 00:00:00 in your zone. Midnight UTC and midnight local time are different epochs unless your offset is zero.
- Can I enter seconds and fractions?
- Yes. The time field accepts hh:mm, hh:mm:ss and fractional seconds up to nine digits, for example 12:00:00.123456789.
- What happens with a nonexistent time like 02:30 during spring forward?
- The result panel says the time does not exist and offers the two possible instants (01:30 EST or 03:30 EDT, expressed in UTC). You choose one; nothing is assumed.