What this calculator does
A date calculator answers one plain question: what date do you land on if you add or subtract a set amount of time from a starting point. Enter a start date, choose a direction and a unit, and the result comes back as a full date, its day of the week, and how many days that is from where you began.
It sounds simple, but calendar arithmetic is not: months have different lengths, years have 365 or 366 days, and counting by hand across a month or year boundary is where mistakes creep in. This calculator does the arithmetic properly, whatever unit you are working in.
The formula
The amount entered is applied in the unit chosen: days and weeks shift the calendar day directly, months and years shift the month or year number and let the calendar resolve the resulting date, which is how a plain-language "add 3 months" is meant to be read.
| Term | Meaning |
|---|---|
| Start date | The date you are counting from. |
| Amount and unit | How much time to add or subtract, and whether that is in days, weeks, months or years. |
| Resulting date | The start date shifted by the amount and unit chosen, in the direction selected. |
The inputs explained
| Field | What to enter |
|---|---|
| Start date | The date you are counting from. Defaults to today. |
| Direction | Whether the amount is added to the start date or subtracted from it. |
| Amount | How much time to shift by, in whatever unit is selected below. |
| Unit | The unit the amount above is measured in. |
When to use it
Working out a deadline or due date
Adding a fixed number of days or weeks to today is the most common way contracts, invoices and project plans state a deadline, and it is easy to miscount by hand near a month boundary.
Checking a date to date calculator style question
Sometimes the question runs the other way: given a start date and a known end date worked out elsewhere, subtracting confirms the gap matches what was expected.
Planning an event a fixed distance out
Adding months or years to a date, such as planning a follow-up appointment six months out, needs the calendar to land on a sensible date even when the start date is near the end of a month.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Where a fixed number of days from today lands, week by week
A series of day counts added to today, each landing on a later date and weekday.
| Days added | Resulting date | Day of the week |
|---|---|---|
| 7 days | 20 Aug 2026 | Thursday |
| 14 days | 27 Aug 2026 | Thursday |
| 30 days | 12 Sep 2026 | Saturday |
| 60 days | 12 Oct 2026 | Monday |
| 90 days | 11 Nov 2026 | Wednesday |
| 180 days | 9 Feb 2027 | Tuesday |
Adding months versus adding weeks from the same start date
The same start date, shifted by whole months against the roughly equal span in weeks.
Questions
How does a date calculator handle months of different lengths?
Adding whole months shifts the month number and lets the calendar resolve the day, so adding 1 month to 31 January can land on a date in early March if February does not have 31 days, rather than being forced onto an invalid 31 February.
Can I calculate the date a set number of weeks ago?
Yes, choose "Subtract" as the direction and "Weeks" as the unit. The same applies to subtracting days, months or years from any start date.
Is this the same as a date to date calculator?
A date to date calculator usually finds the gap between two known dates, which is what the days between dates calculator does. This calculator works the other way: it starts from one date and a length of time, and finds the resulting date.
Does the result account for leap years automatically?
Yes. Adding or subtracting in days, weeks or years always uses the actual calendar, including the extra day in February during a leap year, so the resulting date and weekday are correct across a leap year boundary.
To find the number of days, weeks or months between two dates you already know, use the days between dates calculator. For the date a specific number of days from today, see the days from today calculator.