Count Days Between Two Dates Without an Off-by-One Error
Decide whether the final date counts before comparing a duration, trip or leave request.
Duration and covered dates are different
January 1 to January 2 is one elapsed calendar day. A stay or attendance record covering both named dates may instead need a count of two dates. Neither convention fits every task. Decide what you are counting, then use the end-date option that matches the rule.
A short worked example
From January 5 to January 12, 2026, the ordinary difference is seven days, or one full week. Including January 12 changes the covered-date count to eight. Write the convention beside your answer when another person will use it; otherwise the same inputs can appear to give conflicting results.
Months cannot be replaced by thirty days
Calendar months have different lengths. A years/months/days result uses completed calendar units, while total days measures the entire interval independently. January 31 to February 28 can be one completed month under month-end clamping while still containing 28 elapsed days. Use total days when every day has equal weight.
Choose another tool when necessary
For a Monday–Friday project schedule, use business days and add the actual excluded holiday dates. For an event starting at 15:00, a date-only difference leaves out the hours. Use a dated time conversion if the beginning and end are in different time zones.
Put it into practice
Date Duration Calculator
Find the number of days, weeks, and calendar years, months and days between two dates. Choose whether to include the end date.
Date & calendarBusiness Days Calculator
Count working days between two dates with a choice of weekends and a list of dates to exclude as holidays.
Date & calendarAdd or Subtract Days
Add or subtract a number of calendar days or weeks from a date to find a deadline, follow-up date or project milestone.
Date & calendar