How the result is worked out
Days and weeks are simple: a week is always 7 days, so adding or subtracting either just moves that many whole calendar days, including any leap day the span crosses. Months and years work differently: they move to the same date on the target month, so the length in days can vary. Adding 1 month to 31 January lands on 28 February (or 29 February in a leap year), because February has no 31st.
For example, 90 days after 27 September 2026 is 26 December 2026, a Saturday. That's an exact day count, unlike "6 months," which shifts by a slightly different number of days depending on which calendar months it crosses.
Common questions
How do I find out what date it will be in 90 days?
Leave the starting date as today, enter 90, choose "Days" as the unit and leave the direction set to Add. This is the calculation behind common contract and notice-period terms like "within 90 days of this agreement."
Why does adding a month sometimes land on a different day of the month?
A month is counted to the same day number in the next month, so 31 January plus one month lands on 28 February, or 29 February in a leap year, because those months don't have a 31st. This is the same rule courts and contracts use for "one month's notice."
Can I subtract days instead of adding them?
Yes. Switch the toggle to Subtract and the calculator counts backward from the starting date instead, which is useful for working out when something must have started to be due, or completed, by now.
What's the difference between adding weeks and adding months?
Adding weeks always moves a fixed number of days (a week is always 7 days), while adding months moves to the same date next month regardless of how many days that month has. Six weeks from a date is always 42 days later; six months is a date-to-date jump that can span 180 to 184 days depending on which months are crossed.
Does this account for leap years?
Yes. Adding days or weeks counts real calendar days including 29 February, and adding years lands on the same date one year later, moving a 29 February start to 28 February in a non-leap year.
Is the result affected by time zones?
No. The calculation works in whole calendar days on your device's own calendar, not a specific time of day, so time zone and daylight saving changes don't affect the result.