Date Calculator

What the date calculator does

You have a date, and you need the one that lands a certain stretch of time away from it. Ninety days after a contract is signed. Three months before a due date. Two years from today. That is what this tool is for. You give it a starting date, tell it whether to go forward or back, and enter how many years, months, weeks, and days to move, and it hands you the new date, along with the weekday it falls on.

You can use one of those boxes or all four together, so it copes equally with a simple "what is 45 days from now" and a fiddlier "2 years, 1 month, and 10 days from this date."

How to use it

1. Start Date. The date you are counting from.
2. Operation. Add to move forward in time, or Subtract to move back.
3. Years, Months, Weeks, Days. How far to move. Fill in whichever ones you need and leave the rest at zero.

Press Calculate for the new date, or Reset to clear the boxes.

How the new date is worked out

It applies your numbers to the start date in order, the years first, then the months, then the weeks and days. Adding two years bumps the year by two, adding three months walks the calendar forward three months, and the weeks and days are added on as plain days, a week being seven of them. Subtracting does the same in reverse. Once it has the new date, it also reads off the day of the week, so you can see at a glance whether it lands on a weekend.

An example with real numbers

Say the start date is 15 June 2024, and you add 3 months and 10 days.

• Add 3 months to 15 June, and you reach 15 September.
• Add 10 days to that, and you land on 25 September 2024.

So the resulting date is 25 September 2024, and the tool tells you which weekday that is too. The years and months move you in calendar steps, while the days are simply counted on top.

What happens at the end of a month

There is one case worth understanding, because it surprises people. Months are not all the same length, so adding a month to a date near the end of a month does not always do what you might expect. Take 31 January and add one month. There is no 31 February, so the calendar carries the extra days over into the next month. In a normal year February has 28 days, so the three days past the 28th spill into March, and the result comes out as 3 March, not the 28th of February.

This is not the tool getting it wrong, it is just how calendar months behave once you land on a date that the next month does not have. If you want to be sure of hitting a specific day, it is worth glancing at the result whenever your start date sits near the end of a month.

Where this one earns its keep

Anything that is a fixed stretch of time from a known date. Working out a deadline that is 90 days from the day a document was signed. Finding a milestone date, like the date a baby reaches six months. Planning backward from an event to see when you need to start. Because it does both directions and mixes years, months, weeks, and days freely, it handles the awkward "three months and two weeks before" sort of question that a days-only tool cannot.

Questions people ask

Can it go backward in time?

Yes. Choose Subtract instead of Add and it counts back from your start date, which is how you find a date that is a certain amount of time before a known one.

Why did adding a month to the 31st not land on the 30th or 31st?

Because the next month did not have that day. When you add a month to a date near the end of a long month, any days the shorter month does not have roll over into the month after, so 31 January plus a month becomes early March.

Does it skip weekends?

No. This tool counts every calendar day, weekends included. If you need to move by working days only and skip Saturdays and Sundays, the business days calculator does that.

Does it tell me the day of the week?

Yes. Along with the new date, it shows the weekday it falls on, which is handy when you are checking whether a deadline lands on a weekend.

References

1. U.S. Naval Observatory, Astronomical Applications Department: Leap Years. https://aa.usno.navy.mil/faq/leap_years