Calculate the number of days, weeks, months, or years between two dates. Also add or subtract days from any date.
The simplest way to find days between dates is to convert both dates to a day count from a fixed reference point, then subtract. Our calculator does this automatically for any pair of dates.
Common uses for date calculations: counting days until a deadline or event, calculating someone's age in days, determining contract durations, figuring out when a payment is due, and calculating how many shopping days until a holiday.
Adding months is trickier than adding days because months have different lengths. Adding 1 month to January 31 is ambiguous — some systems use February 28, others use the last day of the target month. Our calculator uses the standard approach of adjusting to the last valid day of the month.
Date arithmetic counts the days separating two calendar dates, accounting for month lengths and leap years (every 4 years, except centuries not divisible by 400). Calculators typically convert each date to a serial day number and subtract.
From 1 January 2026 to 4 July 2026 is 184 days: 30 remaining in January + 28 (February 2026 is not a leap year) + 31 + 30 + 31 + 30 + 4.
Inclusive or exclusive counting? Convention excludes the start date and includes the end date; contract deadlines may define this differently, so read the terms.
Business days? Weekday counts exclude Saturdays and Sundays and usually public holidays, which vary by country.
Why do months differ in length? The Roman calendar's legacy — which is exactly why automated day counting beats mental arithmetic for anything important.