Date Calculator

How to use the date calculator

1. Enter the start date. It defaults to today, so the tool works as a “date from today” calculator straight away.
2. Choose Add to go forward in time or Subtract to go back.
3. Type any combination of years, months, weeks and days. Leave the ones you don’t need at zero.
4. Tick business days only if the Days part should skip Saturdays and Sundays, for a delivery estimate or a notice period counted in working days.

The result shows the new date and its weekday, how many calendar days and weeks it is from the start, the number of business days crossed, the day of the year and the ISO week number. The step-by-step box lists each stage so you can check it by hand.

How the date calculation works

Adding days is simple counting. Each date is converted to a day number, the days are added, and the total is converted back to a calendar date. Because the arithmetic uses whole calendar days, daylight-saving changes cannot push the answer forward or back by a day.

Months and years are different because they are not a fixed number of days. A month can have 28, 29, 30 or 31 days. So the calculator moves the month and year first and keeps the same day of the month:

New date = start date + years and months (same day of month) + weeks × 7 days + days

If that day does not exist in the target month, the date moves back to the last day of the month. This is the same rule Excel’s EDATE function uses. Weeks and days are then added as ordinary days.

Worked examples

All of these start on Tuesday, November 10, 2026, unless noted.

Ninety days from November 10, 2026 lands in week 6 of 2027 and crosses 64 weekdays. If you are counting a 90-day return window or a probation period, check whether your policy counts calendar days or working days, because the two answers are weeks apart.

Month-end dates and leap years

Month arithmetic is where most date mistakes happen:

• January 31 + 1 month gives February 28, 2027, or February 29, 2028 in a leap year.
• August 31 − 6 months gives February 28, 2026.
• February 29, 2028 + 1 year gives February 28, 2029, since 2029 has no leap day.

A leap year has 366 days. Under the Gregorian calendar a year is a leap year if it divides by 4, except century years, which must divide by 400. So 2000 and 2028 are leap years and 2100 is not.

One side effect is that month steps don’t always reverse. January 31 plus one month is February 28, but February 28 minus one month is January 28. If you need a fixed number of days, enter days instead of months.

Business days and deadlines

With the business days option ticked, the Days part counts only Monday to Friday. Starting on a Friday, one business day later is the following Monday. Ten business days after Friday, November 13, 2026 is Friday, November 27, 2026.

Public holidays differ by country, state and employer, so the calculator does not remove them. Add one day for each holiday that falls inside your range. For the number of days between two dates you already know, use the days between dates calculator instead.

Date formulas in Excel and Google Sheets

Spreadsheets store dates as serial numbers, so the same logic works there:

• =A1+90 adds 90 days to the date in A1.
• =A1-30 goes back 30 days.
• =EDATE(A1,6) adds six months and clamps month-end dates.
• =WORKDAY(A1,10) adds ten working days. A third argument can list holiday dates to skip.
• =TEXT(A1,"dddd") shows the weekday name.

Tips

• To find a date of birth from an age, subtract the age in years from today, then check the exact day with the age calculator.
• For pregnancy dates, the due date calculator adds 280 days to the first day of the last period.
• To add hours and minutes rather than days, use the time calculator.