Tue 29 Sep 2026 plus 30 days is Thursday, 29 October 2026.
Day numbers count from 1 January 1970 in the proleptic Gregorian calendar.
Thursday, 29 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.
核验来源：Python dateutil: date(2024,1,31) + relativedelta(months=1) = 2024-02-29 (Excel EDATE agrees)
核验来源：Python 3.8: date(2026,1,15) + timedelta(days=100)
核验来源：Python dateutil: date(2024,2,29) + relativedelta(years=1) = 2025-02-28
核验来源：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.
准确性取决于输入值和方法的假设。十进制运算使用50位有效数字，但估算、数值方法和源数据的精度可能较低；显示时的舍入并不能消除这些限制。 已按独立来源核验的计算示例：7。 例如，“31 January plus one month in a leap year”根据Python dateutil: date(2024,1,31) + relativedelta(months=1) = 2024-02-29 (Excel EDATE agrees)进行核验。
Howard Hinnant — chrono-compatible low-level date algorithms; Microsoft Excel EDATE and WORKDAY functions (same month-end and business-day conventions).
此计算器包含 7 个已解示例，答案来自独立来源。这些示例会在测试套件中运行，你也可以在此运行验证。
计算两个日期之间的天数，也可按年、月、日、周和小时表示；还可排除周末及自定义假日，计算工作日。
The deadline N business days after or before a start date, skipping weekends and the holidays you list, with the calendar days it spans.
实时显示距离任意时区指定日期与时间还剩或已过的天、小时、分、秒，并考虑时钟调整。
根据出生日期计算任意日期的准确年龄，以年、月、日表示，并显示下次生日及以月、周、天和小时计的年龄。