Calendar Date Metrics
Understand exactly what a date range counts, including inclusive days and ISO weeks.
Detailed explanation & worked examples
Elapsed versus inclusive days
January 1 to January 2 is one elapsed day but two inclusive calendar dates. Elapsed days = end − start. Inclusive days = elapsed + 1. Weekday and weekend counts include both endpoints.
Weeks and leap years
A full elapsed week is seven days. Calendar months are not fixed 30-day periods. Gregorian leap years are divisible by four, except century years not divisible by 400. ISO week-year can differ from the calendar year near January and December.
These calculations use civil Gregorian dates in UTC, not local elapsed clock hours. Weekdays are not holiday-adjusted business days.
NIST conversion factors · BIPM SI reference · Week-number explanation
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