Date Calculator
Pick a start date, choose how much to add or subtract, and get the exact resulting date — including its weekday, which day of the year it is, and how far away it is from today.
Why 31 January plus one month is not 31 February
Adding months is not the same as adding 30 days. A month is a calendar unit whose length changes, so the result depends on where you start.
The rule that matches how people read calendars: keep the day number if that day exists in the target month, otherwise use the last day of that month. So 31 January plus one month is 28 February (or 29 in a leap year), not 3 March.
Naive implementations let the date overflow — JavaScript's own setMonth does this by default — which is why some online calculators produce answers that are a few days off for month-end dates. This one clamps to the last valid day instead.
Days, weeks and months are different operations
Adding days is exact: 30 days is always 30 days, regardless of the month. This is what you want for deadlines measured in days.
Adding months is a calendar operation, so 1 January plus one month is 1 February (31 days later), while 1 February plus one month is 1 March (28 days later). If you need a fixed interval, use days.
Adding years follows the same rule, with one extra case: 29 February plus one year lands on 28 February in non-leap years.
Common uses
Contract and notice periods. “30 days from signing” and “one month from signing” give different dates — this tool shows which one you actually meant.
Follow-up scheduling. A 90-day review from a start date, or a 6-week checkpoint, without counting on your fingers.
Age and eligibility checks. Working out the date someone turns a given age, or the last day to file something.
Frequently asked questions
Why does adding one month to 31 January give 28 February?
Because 31 February does not exist. The calculator uses the last valid day of the target month, which is how calendars are read in practice.
Is adding 30 days the same as adding one month?
No. 30 days is a fixed interval; one month varies between 28 and 31 days. They only coincide in 31-day months starting on the 1st.
Does it handle leap years?
Yes. 29 February plus one year becomes 28 February in a non-leap year, and the result panel shows whether the target year is a leap year.
Can I subtract instead of add?
Yes — switch the operator to minus, or use the flip button to swap the sign.
What does 'day of year' mean?
The ordinal position of the date within its year: 1 January is 1, 31 December is 365 (366 in a leap year).
Does it calculate the difference between two dates?
No — this tool adds and subtracts. For the gap between two dates, use the distance-from-today figure or a dedicated date-difference tool.