Date Calculator
Add or subtract days, weeks, months or years from a date and find the resulting date, with the weekday.
How it works
You provide a start date, choose to add or subtract, enter an amount and select a unit — days, weeks, months or years. The tool works out the resulting date and also shows it in long form, with the weekday.
The calculation respects the quirks of the calendar: months with different numbers of days, leap years, and the rollover between months and years. When you add weeks, it multiplies by seven; when you add months or years, it moves through the calendar consistently, without you counting anything by hand.
Because the weekday often matters as much as the date itself — to know whether a deadline falls on a weekend, for example — it appears alongside the result. Everything is calculated in your browser the moment you adjust any field.
When to use
Calculating future or past dates is routine for deadlines and planning. Finding out when a 90-day contract expires, what the date will be six months from now, when the anniversary of an event lands, or what day it was a specific number of days ago.
It's useful for work and legal deadlines, project planning, tracking expiry dates, scheduling appointments and general organization. Knowing in advance which weekday a date falls on helps you avoid booking something important on a Saturday or Sunday, or anticipate a due date landing on a holiday weekend.
Practical examples
A 90-day deadline
A contract signed today with a 90-day term: pick today's date, "add", 90 and "days". The tool shows the exact expiry date and tells you which weekday it falls on — handy so the deadline doesn't slip onto a weekend.
Six months ago
To find the date 6 months ago, just choose "subtract", 6 and "months". The calculation respects the different number of days in each month, with no calendar counting on your part.
Common mistakes
The most common mistake is adding months while assuming every month has 30 days. They don't: February has 28 or 29, and the rest alternate between 30 and 31. So "add one month" isn't the same as "add 30 days" — the tool moves through the real calendar, not fixed blocks.
Another thing to watch is leap years. Every four years (with exceptions), February gains a day. In calculations that cross February 29, ignoring this creates a one-day difference — easy to miss when counting by hand.
There's also the confusion over whether to include the start date in the count. When adding deadlines, some contexts count the starting day itself, others begin the next day. The tool shifts the date by the amount you enter; check which convention your case requires.
Frequently asked questions
How do I add days to a date?
Pick the start date, select "add", enter the number of days and keep the unit on "days". The resulting date appears automatically, along with the weekday.
Does the calculator account for leap years?
Yes. It uses the real calendar, so calculations passing through February 29 in leap years are handled correctly, with no one-day error.
Can I subtract months or years, not just days?
Yes. The unit can be days, weeks, months or years, for both adding and subtracting. Months and years move forward or back through the calendar consistently.
Does the result show the weekday?
Yes. Besides the date, the result includes the long form with the weekday, which helps with planning deadlines and appointments.