gmt-arithmetic

Use GMT's addDate with a unit object, such as addDate("2024-03-15", { days: 5 }). Arithmetic clamps by default, so January 31 plus one month becomes February 29; pass { overflow: "reject" } to get an empty-string sentinel instead.

Use intervalsOverlap from GMT's interval namespace, which treats intervals as half-open [start, end) over instant strings. Touching intervals do not overlap; the older positional intervalsOverlapUtc family is closed and includes shared endpoints, so do not mix the two models.

Build a BusinessCalendar with weekend as ISO weekday numbers and a holidays array, then call businessDaysBetween. The count is start-exclusive and end-inclusive, and GMT bundles no holiday table, so you supply holidays yourself.

Yes, but only through the duration functions: parseDuration, addDuration, normalizeDuration, and diffDateAsDuration. The add* functions take a { unit: number } object, so passing "P5D" to addDate returns the empty-string sentinel.

spanMs and spanNs measure actual elapsed time, while spanWallClock measures what the clock face showed. Across a DST transition a wall-clock day is 23, 24.5, or 25 real hours, so the two measures differ by the size of the shift.

GMT returns correct TC39 results across Temporal's full instant range from -271821-04-20 to +275760-09-13. spanNs and sumIntervals stay exact as bigints past Number.MAX_SAFE_INTEGER, and fractional or unsafe epoch inputs return the sentinel.