Date Calculator

Counting days between dates

The key question is whether the endpoints are included. This calculator reports the elapsed interval, so January 1 to January 10 is 9 days — the number of nights between them, and the count you want for durations and interest.

Inclusive counting, which would give 10 for the same range, is used for things like hotel nights versus days of stay, jury service, and some contractual notice periods. When a deadline is expressed as a number of days, it is worth establishing whether the count includes the starting day.

Business days

The business-day count here excludes Saturdays and Sundays but not public holidays, which vary by country, state, and industry. For shipping estimates, payment terms, or legal deadlines, subtract any holidays that fall inside the range.

Note also that financial markets, courts, and banks each keep their own calendars, which do not always coincide. Payment terms such as net 30 usually mean 30 calendar days rather than business days unless the contract says otherwise.

Why adding months is ambiguous

Adding a month to January 31 has no single correct answer, because February has no 31st. The standard convention, used here, clamps to the last valid day of the target month, so January 31 plus one month is February 28 (or 29 in a leap year).

This makes month arithmetic non-reversible: January 31 plus one month is February 28, but February 28 minus one month is January 28, not the 31st. Software and contracts handle this differently, so for anything consequential — subscription renewals, loan due dates — check which convention applies rather than assuming.

Calendar quirks that affect date math

Two historical and geographic details occasionally matter. The switch from the Julian to the Gregorian calendar removed days from the calendar — 10 days in 1582 in Catholic Europe, 11 in Britain and its colonies in 1752, and as late as 1918 in Russia — so intervals spanning those transitions need care.

Daylight saving transitions also make some local days 23 or 25 hours long. Day-level arithmetic is unaffected, but calculations that mix dates and times across a transition can be off by an hour, which is why software normally computes in UTC and converts only for display.

US federal holidays in 2026

The business-day count above ignores holidays, so subtract the ones that fall in your range. When a federal holiday lands on a Saturday it is observed on the Friday before; on a Sunday, the Monday after.