Add an ISO 8601 duration to a start date or date-time and return the resulting UTC instant. Calendar-aware: months and years honor month lengths (e.g. Jan 31 + P1M clamps to the last day of February). A negative-signed duration subtracts.

Compute the calendar breakdown (years, months, days, hours, minutes, seconds) between two ISO 8601 dates/date-times, plus exact totalSeconds. If end precedes start the result is flagged negative rather than erroring.

Add/subtract business days to a date, or count business days between two dates — skipping weekends and optional holidays.

Convert an ISO datetime into another timezone, with offset, weekday and Unix time.

Return the ordinal day of the year (1..366), the number of days in that year and whether it is a leap year.

Compute the date of Western (Gregorian) Easter Sunday for a year in 1583..9999 using the Anonymous Gregorian computus.

Assemble an ISO 8601 duration string from individual components (years, months, weeks, days, hours, minutes, seconds) and an optional sign. All components must be non-negative and at least one must be non-zero.

Convert a Julian Day Number back to a proleptic-Gregorian date, with the ISO weekday (1=Monday..7=Sunday) and its name.

Render an ISO 8601 duration as an English phrase, e.g. 'P1Y2M' → '1 year and 2 months'. Components are shown in descending order with correct pluralization.

Return the ISO-8601 week-numbering year, week number (1..53) and weekday (1=Monday..7=Sunday) for a Gregorian date. The ISO year can differ from the calendar year around New Year.

Parse an ISO 8601 duration such as 'P1Y2M10DT2H30M' into its components. Returns a normalized ISO string and totalSeconds — but totalSeconds is null when the duration contains years or months, because those are calendar-variable (use /v1/add or /v1/between for exact date math).

Convert a proleptic-Gregorian calendar date to its Julian Day Number (the integer count of days since JDN 0, 1 January 4713 BC in the proleptic Julian calendar).

Return the ISO weekday (1=Monday..7=Sunday) and its name for a Gregorian date.