# Date Calculator - Add or Subtract Days, Weeks, Months or Years from a Date

Page: https://hirantha.me/tools/date-calculator
Part of: Hirantha Sandaruwan Lakshan free tools (https://hirantha.me/tools)
Updated: 2026-10-13

Add or subtract years, months, weeks and days from any date, or count working days, and see the weekday of the result.

This is a free tool that runs in the browser: open the page to use it. File tools process files on the visitor's own device; files are never uploaded or stored. No account, sign-up, payment or watermark.

Start from any date and add or subtract years, months, weeks and days to get the new date and its weekday. Or switch to working days to find the date 10 or 30 working days away, skipping weekends and, if you want, Sri Lanka's public holidays. It runs in your browser and sends nothing anywhere.

In calendar mode the tool applies years and months first, where a year counts as twelve months. A day that does not exist in the month reached becomes that month's last day, so 31 January plus one month is 28 February (29 in a leap year) and 29 February plus one year is 28 February. Subtracting follows the same rule backwards.

Weeks and days are applied after the months, as plain calendar days, so the order matters. Adding 1 month and 1 day to 30 January 2026 gives Sunday 1 March 2026: the month first makes 28 February, then one more day. Adding the day first would give 31 January, and a month after that is 28 February.

Working-day mode steps forward (or back) one day at a time and counts only Monday to Friday. The start date is not counted, so one working day after a Friday is the next Monday, and a start date on a weekend begins with the next working day. Zero working days gives the start date. With the holiday option on, Sri Lanka public holidays on weekdays are skipped too, for 2026 and 2027 only. Outside those years only weekends are skipped, and the tool warns you.

The date is a calendar date, with no time of day and no time zone, so daylight saving cannot move the answer. 'Today' means today in Sri Lanka (Asia/Colombo, UTC+5:30). The tool covers the years 1583 to 9999.

Q: What is the date 90 days from today?
A: Leave the start date as today, choose Add, and enter 90 in the Days box. For example, 90 days after 13 October 2026 is Monday 11 January 2027.

Q: What is 31 January plus one month?
A: 28 February 2026 in 2026, and 29 February 2028 in the leap year 2028. February has no 31st, so the tool uses its last day. Some programs roll the extra days over into March instead, so another tool may give a different answer.

Q: How do I add working days to a date?
A: Choose 'Working days (Mon to Fri)', enter the number and pick whether to skip Sri Lanka public holidays as well. The start date is not counted, and Saturdays and Sundays are always skipped.

Q: Is a month 30 days?
A: No. The tool adds calendar months. Adding 1 month to 15 January 2026 gives 15 February, 31 days later, but adding it to 15 February gives 15 March, 28 days later.

Q: Does adding days count the start date?
A: No. One day after 1 January is 2 January. In working-day mode the start date is not counted either.

Q: What happens if I add one year to 29 February?
A: You get Friday 28 February 2025 for 29 February 2024, the last day of February in the next year. Add four years and you reach Tuesday 29 February 2028.

Q: Are Sri Lanka public holidays included?
A: Only when you choose them, in working-day mode, and only for 2026 and 2027, taken from the Central Bank of Sri Lanka's lists of public and bank holidays. The lists mark only some of them as mercantile holidays, so a private employer's days off can differ. Outside those years the tool skips weekends only and says so.

Sources: Leap year - Wikipedia (https://en.wikipedia.org/wiki/Leap_year); Gregorian calendar - Wikipedia (https://en.wikipedia.org/wiki/Gregorian_calendar); Central Bank of Sri Lanka: bank holidays for 2026 (https://www.cbsl.gov.lk/en/about/about-the-bank/bank-holidays-2026); Central Bank of Sri Lanka: bank holidays for 2027 (https://www.cbsl.gov.lk/en/about/about-the-bank/bank-holidays-2027); Howard Hinnant: chrono-Compatible Low-Level Date Algorithms (https://howardhinnant.github.io/date_algorithms.html)

Related tools:

- [Days between two dates calculator](https://hirantha.me/tools/date-difference-calculator.md): Count the days between two dates in weeks, months and years, with business days, an end-date option and Sri Lanka holidays left out if you like.
- [Day of the week calculator](https://hirantha.me/tools/day-of-week-calculator.md): Find the weekday of any date from 1583 to 9999, plus the day of the year, ISO week number, leap year check and days left in the year.
- [Age calculator (years, months and days)](https://hirantha.me/tools/age-calculator.md): Exact age in years, months and days, days, weeks and hours lived, the next birthday and its weekday, and the age on any date.
- [Sri Lanka public holidays 2027 & 2026](https://hirantha.me/tools/sri-lanka-holidays.md): Every gazetted public, bank and mercantile holiday and Poya day, with the next holiday, long weekends and a working-day counter.