Date math
Date Calculator: Add or Subtract Days
Add or subtract days, weeks, months or years from any date and get the resulting date and weekday — with correct month-end and leap-year handling.
How to use the date calculator
- Set the start date (today by default).
- Choose Add or Subtract.
- Type any combination of years, months, weeks and days. Leave a box at zero to ignore it.
- Read the resulting date, its weekday, its ISO date and ISO week, and the net number of days moved. Everything recalculates as you type.
How date arithmetic handles months and leap years
Adding days is unambiguous — 45 days after a date is always exactly 45 days later. Adding months and years is where tools disagree, because months have different lengths. This calculator uses the widely accepted rule:
- Years and months first. The tool moves the month (and year) and keeps the same day-of-month when it exists. When it does not — for example the 31st in a 30-day month, or the 29th in a non-leap February — the day is clamped to the last day of the target month. So January 31 plus one month is February 28 (or February 29 in a leap year), and the tool flags when this clamping happened.
- Weeks and days next. After the month move, whole weeks (seven days each) and individual days are added or subtracted. Because a week is exactly seven days, adding weeks always lands on the same weekday.
Applying months before days is the standard order used by spreadsheets and calendar apps, and it keeps results predictable: "one month and ten days from now" first finds the same day next month, then counts ten more days.
People reach for date arithmetic constantly: a returns window that ends "30 days after delivery," a contract that starts "the first of next month," a reminder set "six weeks out," or a warranty that runs "two years from purchase." Each is a start date plus an offset, and each depends on the month-end and leap-year rules above to land on a real calendar date. When the offset should skip weekends instead of counting raw days, count it with the business-days option in the days between dates tool rather than adding plain days here.
All arithmetic is done on local date components using an exact day-number conversion, never a UTC timestamp, so results do not shift by a day near midnight or across time zones. To measure the gap between two known dates instead of projecting forward, use the days between dates calculator.
Frequently asked questions
How do I add days to a date?
Enter a start date, leave the direction on Add, type the number of days, and read the resulting date. It updates instantly and shows the weekday too.
What happens when I add a month to January 31?
February has no 31st, so the result is clamped to the last day of February — the 28th, or the 29th in a leap year. Adding months always lands on a real date, and the tool notes when clamping occurred.
In what order are years, months and days applied?
Years and months first (which may clamp the day to the month end), then weeks and days. This matches most calendar software.
Can I subtract time to find a past date?
Yes — switch the direction to Subtract to count backwards.
Does adding 7 days always give the same weekday?
Yes. Any whole number of weeks, or any multiple of seven days, lands on the same weekday. The tool shows the weekday so you can confirm.
DateSpan is a free calendar utility provided for general informational purposes only. For contractual or legal deadlines, confirm the counting method that applies to your situation.