What happens when I add a month to the 31st?

The result keeps the same day number when the target month has it, and lands on the last day of the month when it does not. So 31 January plus one month is 28 February (29 in a leap year), 31 January plus two months is 31 March, and 30 November plus three months is 28 February. The line under the result says which of the two happened, so a clamped date is never a silent surprise.

How are leap years handled?

By the Gregorian rule: every fourth year is a leap year except century years, unless the century is divisible by 400, so 2000 and 2024 are leap years and 1900 and 2100 are not. Adding one year to 29 February gives 28 February, adding four gives 29 February again, and a span that crosses a 29th of February counts it as a day like any other.

What is the difference between days and weekdays?

Days walk straight through the calendar, weekends included. Weekdays are Monday to Friday only: as a unit, they step over Saturdays and Sundays, so Friday plus one weekday is Monday; as a count, they tell you how many Monday-to-Friday days lie in the span. Public holidays are not known here. For a count with US federal or UK bank holidays removed, use the business days calculator.

Does the count include both dates?

The day count is the nights between the two dates, so Monday to the next Monday is 7 and 1 September to 30 September is 29. The weekday and weekend-day counts include both dates, the way "working days in September" counts the 1st and the 30th: September 2026 has 22 weekdays and 8 weekend days, 30 in all. The page says which convention each figure uses under the result.

Do time zones or daylight saving affect the result?

No. Every date is treated as a calendar date, not a moment on a clock, so a clock change during the span can never add or lose a day. The only place your device's time zone matters is the Today button, which reads the date as your calendar shows it.

How is the week number worked out?

By ISO 8601: weeks start on Monday and week 1 is the week that holds 4 January. A week belongs to the year that holds its Thursday, so 1 January 2027 falls in week 53 of 2026. The week number tool shows the US convention alongside if you need that instead.

Can I count backwards?

A negative whole-number change subtracts: -30 days goes back thirty calendar days, which may differ from one calendar month. In Days between mode an end date before the start counts backwards and the page says so. Gregorian calendar dates from year 0001 to 9999 are accepted; historical calendar adoption and Julian dates are not modeled.

How do I find a date 30, 60 or 90 days from a start date?

Choose Add or subtract, enter the start date, select Days and type the signed whole-number offset. November 7, 2024 plus 90 days is February 5, 2025. Minus 30 goes back thirty days. Use Weeks for a fixed seven-day multiple, or Months for calendar-month arithmetic; six calendar months need not equal 180 days.

How do I add calendar weeks, months or years?

Choose the unit and enter a whole-number offset. Starting January 31, 2024, four weeks ends February 28, one month ends February 29, six months ends July 31, 52 weeks ends January 29, 2025 and one year ends January 31, 2025. A fixed number of weeks is not the same as a calendar year.

How do I count elapsed days since a date or personal milestone?

Open Days between, enter the supplied past date as Start, then choose End today. The End field uses the date on your device. The result shows calendar nights, weeks/days, calendar months/days and years/months/days. A supplied milestone, clean or sobriety date can be counted the same way. It is a calendar count and does not keep a journal, track relapses or infer the start date. No date or result is saved.

How are years, months and days between dates calculated?

The tool first counts whole calendar months with the stated month-end clamp rule, then the days left over. The same whole-month count is grouped into twelve-month years and remaining months. January 31, 2024 to March 2, 2025 is 13 whole months and two days, displayed as one year, one month and two days. This does not divide elapsed days by an average month or year. Backward spans use the same earlier-to-later breakdown with the direction labelled.

Why can a same-date span show zero days and one weekday?

Days counts the nights between endpoints, so the same date has zero nights. Weekdays and weekend days are inclusive counts and include that one date: a Friday contributes one weekday, a Saturday one weekend day. Use the separate Business Days Calculator when you need an explicit inclusion choice or its supported public-holiday sets.

Can I find the weekday or ISO week of a date?

Every result shows its weekday and ISO week. To inspect a supplied date without shifting it, select Days with an offset of zero. The dedicated Week Number tool compares ISO and US week-number conventions.

Are these dates saved, and can I copy the result?

Dates and offsets are not saved or uploaded. Today is read from local calendar fields after the page loads; the calculations use civil Gregorian dates rather than timezone-dependent elapsed hours. Copy uses the native clipboard for the current result or complete summary. A blocked copy leaves the result visible and explains how to copy it manually.

Date Calculator

Add or subtract days, weeks, months, years or weekdays from a date, or count the days between two dates, with the day of the week, the day of the year and the ISO week number in the answer. Month arithmetic states its rule: 31 January plus one month is 28 February, and the page says so instead of rolling quietly into March.

Result dateLoading today's date.

Mode

Start

Change

Unit

Adding a month keeps the day number when it can. 31 January plus one month is 28 February (29 in a leap year), and the line under the result says when that happened instead of rolling quietly into March. Weekday counts drop Saturdays and Sundays only. For public holidays use the business days calculator; for hours and minutes between two moments, the duration calculator.

Calendar offsets from January 31, 2024

Each row starts from the same Gregorian date and uses the same unit/offset rule as the calculator. Enter the start, amount and unit above to try another case.

Offset	Result date	Rule
30 days	March 1, 2024	Calendar days, including leap day
4 weeks	February 28, 2024	Exactly 28 calendar days
1 month	February 29, 2024	Month-end clamp in a leap year
60 days	March 31, 2024	Calendar days
6 months	July 31, 2024	Same day number in target month
52 weeks	January 29, 2025	Exactly 364 calendar days
1 year	January 31, 2025	Same day number one calendar year later

Common questions

What happens when I add a month to the 31st?: The result keeps the same day number when the target month has it, and lands on the last day of the month when it does not. So 31 January plus one month is 28 February (29 in a leap year), 31 January plus two months is 31 March, and 30 November plus three months is 28 February. The line under the result says which of the two happened, so a clamped date is never a silent surprise.
How are leap years handled?: By the Gregorian rule: every fourth year is a leap year except century years, unless the century is divisible by 400, so 2000 and 2024 are leap years and 1900 and 2100 are not. Adding one year to 29 February gives 28 February, adding four gives 29 February again, and a span that crosses a 29th of February counts it as a day like any other.
What is the difference between days and weekdays?: Days walk straight through the calendar, weekends included. Weekdays are Monday to Friday only: as a unit, they step over Saturdays and Sundays, so Friday plus one weekday is Monday; as a count, they tell you how many Monday-to-Friday days lie in the span. Public holidays are not known here. For a count with US federal or UK bank holidays removed, use the business days calculator.
Does the count include both dates?: The day count is the nights between the two dates, so Monday to the next Monday is 7 and 1 September to 30 September is 29. The weekday and weekend-day counts include both dates, the way "working days in September" counts the 1st and the 30th: September 2026 has 22 weekdays and 8 weekend days, 30 in all. The page says which convention each figure uses under the result.
Do time zones or daylight saving affect the result?: No. Every date is treated as a calendar date, not a moment on a clock, so a clock change during the span can never add or lose a day. The only place your device's time zone matters is the Today button, which reads the date as your calendar shows it.
How is the week number worked out?: By ISO 8601: weeks start on Monday and week 1 is the week that holds 4 January. A week belongs to the year that holds its Thursday, so 1 January 2027 falls in week 53 of 2026. The week number tool shows the US convention alongside if you need that instead.
Can I count backwards?: A negative whole-number change subtracts: -30 days goes back thirty calendar days, which may differ from one calendar month. In Days between mode an end date before the start counts backwards and the page says so. Gregorian calendar dates from year 0001 to 9999 are accepted; historical calendar adoption and Julian dates are not modeled.
How do I find a date 30, 60 or 90 days from a start date?: Choose Add or subtract, enter the start date, select Days and type the signed whole-number offset. November 7, 2024 plus 90 days is February 5, 2025. Minus 30 goes back thirty days. Use Weeks for a fixed seven-day multiple, or Months for calendar-month arithmetic; six calendar months need not equal 180 days.
How do I add calendar weeks, months or years?: Choose the unit and enter a whole-number offset. Starting January 31, 2024, four weeks ends February 28, one month ends February 29, six months ends July 31, 52 weeks ends January 29, 2025 and one year ends January 31, 2025. A fixed number of weeks is not the same as a calendar year.
How do I count elapsed days since a date or personal milestone?: Open Days between, enter the supplied past date as Start, then choose End today. The End field uses the date on your device. The result shows calendar nights, weeks/days, calendar months/days and years/months/days. A supplied milestone, clean or sobriety date can be counted the same way. It is a calendar count and does not keep a journal, track relapses or infer the start date. No date or result is saved.
How are years, months and days between dates calculated?: The tool first counts whole calendar months with the stated month-end clamp rule, then the days left over. The same whole-month count is grouped into twelve-month years and remaining months. January 31, 2024 to March 2, 2025 is 13 whole months and two days, displayed as one year, one month and two days. This does not divide elapsed days by an average month or year. Backward spans use the same earlier-to-later breakdown with the direction labelled.
Why can a same-date span show zero days and one weekday?: Days counts the nights between endpoints, so the same date has zero nights. Weekdays and weekend days are inclusive counts and include that one date: a Friday contributes one weekday, a Saturday one weekend day. Use the separate Business Days Calculator when you need an explicit inclusion choice or its supported public-holiday sets.
Can I find the weekday or ISO week of a date?: Every result shows its weekday and ISO week. To inspect a supplied date without shifting it, select Days with an offset of zero. The dedicated Week Number tool compares ISO and US week-number conventions.
Are these dates saved, and can I copy the result?: Dates and offsets are not saved or uploaded. Today is read from local calendar fields after the page loads; the calculations use civil Gregorian dates rather than timezone-dependent elapsed hours. Copy uses the native clipboard for the current result or complete summary. A blocked copy leaves the result visible and explains how to copy it manually.

Dates are calendar arithmetic on the civil calendar, so daylight saving never shifts a result. Adding a month lands on the same day number when it exists and the last day of the month when it does not, and the page says which happened. Weekday counts exclude Saturdays and Sundays only; public holidays are not known.