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Date Calculator

Add or subtract days, weeks, months, or years from a specific date.

What this tool does

Add or subtract days, weeks, months, or years from a specific date. You fill in these fields: Start date, Operation, Years, Months, Weeks, and Days. It shows resulting date, day of the week, and days from the start date.

Inputs

  • Start date: A date. Starts as today's date. Defaults to today. Change it to count from another day.
  • Operation: Choose one: Add, Subtract. Starts as Add.
  • Years: A whole number. Accepts 0 to 9,000. Starts at 0.
  • Months: A whole number. Accepts 0 to 100,000. Starts at 0.
  • Weeks: A whole number. Accepts 0 to 500,000. Starts at 0.
  • Days: A whole number. Accepts 0 to 3,000,000. Starts at 0.

How to use

  1. Pick a date for "Start date".
  2. Choose Add or Subtract for "Operation".
  3. Enter the years.
  4. Enter the months.
  5. Enter the weeks.
  6. Enter the days.
  7. Read the result: resulting date, day of the week, and days from the start date. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • Start date: January 31, 2023
  • Operation: Add
  • Years: 0
  • Months: 1
  • Weeks: 0
  • Days: 0

Results:

  • Resulting date: Tuesday, February 28, 2023
  • Day of the week: Tuesday
  • Days from the start date: 28 days

How this is worked out: Add 1 month to Jan 31: February 2023 has 28 days, so the day is clamped to Feb 28. Jan 31 to Feb 28 = 28 days. Jan 1 2023 is a Sunday; Feb 28 is day 31+28 = 59, so 58 days later; 58 mod 7 = 2, Sunday + 2 = Tuesday.

Example 2. Inputs:

  • Start date: March 15, 2023
  • Operation: Subtract
  • Years: 0
  • Months: 1
  • Weeks: 0
  • Days: 10

Results:

  • Resulting date: Sunday, February 5, 2023
  • Day of the week: Sunday
  • Days from the start date: -38 days

How this is worked out: Subtract 1 month: Mar 15 to Feb 15 (28 days earlier). Subtract 10 days: Feb 5. Total -38. Jan 1 2023 is a Sunday; Feb 5 is day 36, 35 mod 7 = 0, so Sunday.

Example 3. Inputs:

  • Start date: December 25, 2022
  • Operation: Add
  • Years: 0
  • Months: 0
  • Weeks: 2
  • Days: 7

Results:

  • Resulting date: Sunday, January 15, 2023
  • Day of the week: Sunday
  • Days from the start date: 21 days

How this is worked out: 2 weeks + 7 days = 21 days. Dec 25 + 6 days = Dec 31, + 15 more = Jan 15 2023. Jan 1 2023 is a Sunday and Jan 15 is 14 days later, so Sunday.

Example 4. Inputs:

  • Start date: October 8, 2026
  • Operation: Add
  • Years: 0
  • Months: 0
  • Weeks: 0
  • Days: 0

Results:

  • Resulting date: Thursday, October 8, 2026
  • Day of the week: Thursday
  • Days from the start date: 0 days

How this is worked out: Every box empty counts as zero, so the date does not move. Jan 1 2026 is a Thursday; Oct 8 is day 31+28+31+30+31+30+31+31+30+8 = 281, 280 days later; 280 mod 7 = 0, so Thursday.

Where this comes from

Add to or subtract from a date. The years, months, weeks and days are added to the start date, or all taken away from it when subtracting.

Add a length of time to a date. Years and months are added first, keeping the day of the month or the last day of a shorter month. Weeks and days are then added as whole days. Source: The Gregorian calendar: month lengths and the leap year rule (every fourth year, except century years not divisible by 400).

Limitations

  • Works on calendar dates only, with no time of day.
  • Uses the Gregorian calendar for every date, including dates before it was adopted.
  • Enter whole numbers only; fractions of a month or year are not accepted.

FAQs

Q: What happens when adding months lands on a day that does not exist?
A: The result moves to the last day of that month. January 31 plus one month is February 28 (or 29 in a leap year).
Q: In what order are the parts applied?
A: Years and months are applied first, together, then weeks and days. Subtracting reverses the direction of every part.
Q: Can I leave a part at zero?
A: Yes. Each of years, months, weeks and days starts at 0, and a part left at 0 does not move the date.
Q: Does it use my time zone?
A: No. It works on calendar dates only, so the answer does not shift with your time zone or the time of day.

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