Date Calculator
Add or subtract days and months, or count the days between two dates.
Adding a month keeps the day where it exists and clamps where it does not, so 31 January plus one month is 28 February.
Two questions come up constantly and neither is comfortable to do in your head. What is the date ninety days from now, and how many days are there between these two dates.
Both get fumbled for the same reason. Months are not the same length, leap years move everything by a day, and counting on your fingers across a year boundary is how people end up a day out on a contract.
Add or subtract years, months and days in one go and the result comes back as a full date with the day of the week attached. Switch to the second mode and you get the gap between two dates in days, in calendar years and months, and in hours.
How dates are added and counted
When you add, years go on first, then months, then weeks and days. The order matters because months are different lengths, and a month keeps the same day number where it exists.
When you count the days between two dates, the result is the number of days you move forward to get from one to the other. The start date is not counted and the end date is, so 1 March to 31 March is 30 days. If you want both ends included, add one.
A worked example
Ninety days from Thursday 24 September 2026 is Wednesday 23 December 2026. Three months from the same date is Thursday 24 December, a day later, because those three months add up to 91 days rather than 90.
The difference matters in contracts and notice periods, where 90 days and three months are often treated as the same thing and are not.
Mistakes that cause off-by-one errors
Counting both ends by accident. A course that runs from 1 to 5 June has 5 days of classes, but the difference between the dates is 4. Decide which you need before you count.
Assuming every deadline counts the same way. Many rules leave out the day of the triggering event and include the last day. US federal court deadlines work this way, and so do many contracts, so check the wording before relying on a count.
Using calendar days for a working-day deadline. For anything measured in working days, use the business days calculator, which skips weekends and holidays.
A date on the calendar is not a plan.
Knowing the deadline is the easy part. PomoTodo turns it into tasks with time behind them, so you can see whether the work actually fits before the date arrives rather than after.
Free plan tracks unlimited focus time across unlimited tasks. No card required.
More tools
Exact age in years, months and days, plus the next birthday.
Working days between dates, or when a deadline lands.
Weeks from a date, weeks between dates, and week numbers.
Days, hours and minutes until whatever you are waiting for.
Questions
How do I add days to a date?
Pick the start date, choose add, and type the number of days. Years, months and days can all be used at once, and the result appears as a full date including the day of the week.
How many days are between two dates?
Switch to the days between mode and pick both dates. You get the total in days, the same gap expressed in calendar years, months and days, and the equivalent in hours.
What happens when I add a month to the 31st?
The day is kept where it exists and clamped where it does not, so 31 January plus one month is 28 February, or 29 February in a leap year. This is how calendar apps behave and it keeps repeated additions predictable.
Are leap years handled?
Yes. Every count runs on real calendar dates, so 29 February is included whenever it falls inside the range and the century rule is applied correctly.