The date a number of days, weeks, months, years or business days after or before a start date, with its weekday and the calendar days moved.
Sun 27 Sep 2026 plus 30 days is Tuesday, 27 October 2026.
Day numbers count from 1 January 1970 in the proleptic Gregorian calendar.
Tuesday, 27 October 2026, 30 calendar days later.
Days and weeks are added to the start date's day number, a running count of days since 1 January 1970, so month lengths and leap years take care of themselves. Months and years move the month and keep the day of the month; when that day doesn't exist in the target month, the result is the month's last day, the same rule as Excel's EDATE. Business days are counted one at a time from the day after the start, stepping over the chosen weekend days.
Typical uses are due dates, return and cooling-off windows, visa and licence expiry dates and medication courses. 100 days after 15 January 2026 is Saturday, 25 April 2026, and 5 business days after Friday, 16 January 2026 is the following Friday.
Month steps can't always be undone: 31 March 2026 minus one month is 28 February, and 28 February plus one month is 28 March. Public holidays are not skipped here; the working-day deadline calculator takes a holiday list.
Checked against: Python dateutil: date(2024,1,31) + relativedelta(months=1) = 2024-02-29 (Excel EDATE agrees)
Checked against: Python 3.8: date(2026,1,15) + timedelta(days=100)
Checked against: Python dateutil: date(2024,2,29) + relativedelta(years=1) = 2025-02-28
Checked against: Python dateutil: date(2026,3,31) − relativedelta(months=1) = 2026-02-28
Enter today as the start date, choose Add, type 90 and pick Days. Counting starts on the day after the start date, so 90 days from 1 January 2026 is 1 April 2026: January, February and March hold 31 + 28 + 31 = 90 days. In a leap year the same count from 1 January ends on 31 March, because February has 29 days.
28 February, or 29 February in a leap year such as 2024. A day of the month that doesn't exist in the target month is moved back to that month's last day, which is how Excel's EDATE and Python's dateutil relativedelta behave. The same rule makes 31 March minus one month 28 February 2026, and 31 August plus one month 30 September.
Count forward from the day after the start date and skip each weekend day. Friday, 16 January 2026 plus 5 business days is Friday, 23 January, 7 calendar days later; 10 business days before 15 January 2026 is 1 January. Public holidays are not skipped in this calculator, whereas Excel's WORKDAY(start, days, holidays) skips any holidays you list.
Only when the year ahead has no 29 February in it. From 15 January 2027 one year later is 15 January 2028, 365 days on, but from 1 March 2027 one year later is 1 March 2028, 366 days on, because 29 February 2028 falls in between. 29 February 2024 plus one year becomes 28 February 2025 under the month-end rule.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “31 January plus one month in a leap year” is checked against Python dateutil: date(2024,1,31) + relativedelta(months=1) = 2024-02-29 (Excel EDATE agrees).
Howard Hinnant — chrono-compatible low-level date algorithms; Microsoft Excel EDATE and WORKDAY functions (same month-end and business-day conventions).
7 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
Days between two dates, also as years, months and days, weeks and hours, plus business days after weekends and your own holidays.
The deadline N business days after or before a start date, skipping weekends and the holidays you list, with the calendar days it spans.
Countdown to a date: days, hours, minutes and seconds until (or since) a date and time in any time zone, ticking live and allowing for clock changes.
Exact age from a date of birth in years, months and days on any date, with the next birthday and age in months, weeks, days and hours.
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