How the date math works
Pick a start date, choose whether to add or subtract, enter an amount, and select a unit. The calculator shifts the date by that amount and shows the resulting date, its weekday, and the plain year-month-day value.
Days and weeks are simple counts - one week is seven days. Months and years are trickier because they vary in length. When the start day does not exist in the target month, the result is clamped to the last valid day. So January 31 plus one month becomes February 28 (or February 29 in a leap year), and a February 29 date plus one year becomes February 28 in a non-leap year.
Common ways to use it
- Find the date a set number of days from today
- Count forward to a deadline a few weeks or months away
- Count backward to see when something started
- Check which weekday a future date falls on
Related date tools
To count the number of days between two dates instead of shifting one date, use the date duration calculator. To work out an age or anniversary in years, months, and days, try the age calculator.
Frequently asked questions
How do I add days to a date?
Enter your start date, choose Add, type the number of days, and leave the unit on Days. The calculator returns the resulting date and its weekday. To go backward in time, choose Subtract instead.
What date is 90 days from today?
Set the start date to today, choose Add, enter 90, and keep the unit on Days. The result card shows the exact date 90 days out and which weekday it falls on. You can change the start date to count 90 days from any other day.
How does adding months handle the end of the month?
Months are not all the same length, so when the start day does not exist in the target month the result is clamped to the last day of that month. For example, January 31 plus one month lands on February 28 (or February 29 in a leap year), and the same clamping applies to years for dates on February 29.
Can I subtract time too?
Yes. Choose Subtract to count backward from your start date by days, weeks, months, or years. A negative amount flips the direction, so subtracting -5 days is the same as adding 5 days.
Does it count weekends and business days?
It counts calendar days, so weekends and holidays are included in the total. To count the number of days between two specific dates instead, use the date duration calculator.