About Day Delta

Day Delta is a free set of date calculators. Every result is computed with plain Gregorian calendar arithmetic in UTC, which avoids daylight saving errors, and runs in your browser, so the dates you enter are not sent anywhere.

Rules we apply

Counting	Start date is day 0. “Days between” counts midnights; tick “include both” to count both ends.
Months	Same day of month if it exists, otherwise the last day of the target month.
Business days	Monday to Friday. Optionally excludes the 11 U.S. federal holidays at their observed dates.
Server “today”	Shown in U.S. Eastern time. Your browser replaces it with your own date.

Sources

Federal holiday rules follow the U.S. Office of Personnel Management list and 5 U.S.C. 6103. ISO week numbers follow ISO 8601.

Last reviewed 2026-10-27.