Midpoint between two dates

Classic checks include midnight-to-midnight on consecutive days landing at noon, a one-second span landing at the half-second, and identical endpoints returning the same instant.

What an instant midpoint means and what the response contains

The calculator validates that both days exist on the Gregorian calendar—including leap days only in leap years—converts each input to a UTC epoch millisecond count using proleptic civil arithmetic, then sets midpoint_epoch_ms to the floor of the sum of the two epochs divided by two so that an odd total of milliseconds still yields a single deterministic half-millisecond choice. The response echoes the original start and end strings for auditability, returns midpoint as a UTC ISO string with three fractional digits and a trailing Z, reports start_epoch_ms, end_epoch_ms, and midpoint_epoch_ms for numeric pipelines, and includes span_ms as the absolute distance plus signed_span_ms as end minus start so you can see direction when the second instant is earlier than the first. Rejected cases include a missing start or end field, strings that are not ISO-8601, impossible months or days such as 2023-02-29, invalid clock components, and years outside one through 9999.

How start and end are read and how the midpoint stays reproducible

Each string is trimmed; date-only values become midnight UTC; datetimes may use T or a space between date and time; fractional seconds are accepted and truncated to whole milliseconds before averaging so sub-millisecond noise does not invent a second midpoint. Offsets such as plus zero two colon zero zero or minus zero five colon zero zero are converted into UTC by subtracting the offset minutes from the local civil epoch, so a New York evening and a Paris morning that refer to the same absolute interval still average correctly in UTC. The preferred first field is start, but aliases from, date1, a, and begin are accepted so spreadsheets, query strings, and typed forms can reuse the same contract without renaming columns. The preferred second field is end, with aliases to, date2, b, and finish for the same reason. The midpoint is then reformatted with the inverse civil_from_days mapping so the printed UTC string matches the numeric midpoint_epoch_ms exactly. Identical inputs yield the same instant as midpoint; reversing the pair leaves the midpoint and the absolute span unchanged and only flips the sign of signed_span_ms.

Where midpoint datetime arithmetic fits in planning, science, and software

On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split—free interactive exploration and a priced, identical API—matches the rest of the KIT tier-A catalog and lets you move from a one-off check to a pipeline without changing the mathematical result.