Add Days Calculator

Add or subtract days from a date.

Start date

Days to add

Use a negative number to count backwards

Results update as you type.

Resulting date

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Weekday—

How it works

An add days calculator takes a starting date and a number of days and returns the date that lands that many days later, along with the weekday it falls on. Enter a negative number and it counts backwards instead, so the same tool answers both "what date is 45 days from now" and "what date was 45 days ago." It works from a continuous day count rather than nudging the day-of-month field, so it steps cleanly over month ends and year boundaries without you tracking how long each month is. This matters wherever a deadline is defined as an offset. Payment terms of net-30, a 90-day return window, a 14-day cancellation right, an incubation period, or a project milestone stated as "start plus 60 working-adjacent days" all reduce to adding days to a known date. Doing it by hand across February or a year-end is exactly where off-by-one and wrong-month mistakes appear, and a single miscounted day can void a return or miss a filing. The weekday output is often as useful as the date itself, since a deadline that lands on a Saturday or Sunday usually rolls to the next working day in practice. Contract administrators, logistics planners, HR teams and anyone counting toward an event use an add days calculator to turn an offset into a firm calendar date they can put on a schedule.

Formula

endDate = startDate + days (whole days), evaluated on the UTC calendar

The start date is converted to a single continuous count of days (via a UTC timestamp), the offset is added, and the total is converted back to a calendar date. Because a running day index has no notion of month length, every 30-day and 31-day month and every leap day is absorbed automatically. The weekday comes from the result's position in the seven-day cycle, and negative offsets simply subtract, so counting backwards needs no special case.

Source:ISO 8601 / Gregorian calendar

We review all our tools from time to time to ensure accuracy.

Worked example

Start date:2026-01-01Days to add:10

Ten days after 1 January 2026 is 11 January 2026, which is a Sunday.

How to use this calculator

1
Enter your values
Enter start date and days to add.
2
Adjust the options
Fine-tune any options to match your situation.
3
Read your result
Resulting date updates instantly as you type — no button to press.

Frequently asked questions

How do I subtract days instead of adding them?

Enter the number of days as a negative value. Typing -45 in the days field returns the date 45 days before the start date, along with its weekday. The calculator uses the same continuous day count for both directions, so subtracting across a month start or a year boundary is handled exactly the same way as adding, with no separate mode to switch on.

Does it count the start date as day one?

No. The start date is day zero, so adding one day gives the following calendar date, not the start date itself. If your rule counts the start date inclusively, for example a warranty that runs from and including the purchase day, subtract one from the number of days you enter. Whether to count inclusively depends on the wording of your deadline or contract.

Is the weekday accurate across leap years?

Yes. The result is derived from a continuous day count that already includes any 29 February in the range, so the weekday reflects the true position in the seven-day cycle. There is no separate leap-year adjustment to apply. A span that crosses a leap day will shift the weekday by the extra day automatically, which is one reason manual counting so often lands on the wrong weekday.

Does it skip weekends or holidays?

No. This calculator adds calendar days, so weekends and public holidays are included in the count and the result can land on any day of the week. If you need to add working days only, use a business-days tool instead. The weekday output helps here: if your deadline falls on a Saturday or Sunday you can roll it forward to the next working day yourself.

Why compute the date in UTC?

Working on the UTC calendar keeps the result stable regardless of the timezone or daylight-saving rules of whoever runs the calculator. Because only whole calendar days are involved, there is no hour component to shift, so the date you get is the same everywhere. This determinism is what lets the same offset produce the same answer for every user rather than varying by an hour near midnight.