How this calculator works
Counting in days or weeks moves along the calendar exactly, one day at a time, so weekends, statutory holidays and leap days are all ordinary steps. Counting in months or years uses calendar months instead of a fixed 30 days: January 31 plus one month is the last day of February, and a February 29 start plus one year lands on February 28 in a non-leap year. The reverse direction reports the gap between two dates in exact days, in weeks, and in whole calendar months.
Formula
For day and week units, deadline = start_date + sign x amount x 7 (days use a factor of 1). For month units, deadline = start_date moved by sign x amount calendar months, with the day clamped to the shorter month. For year units the month shift is sign x amount x 12. In the reverse direction, days = O(deadline) - O(start_date), where O is the day number counted from 0001-01-01.
Worked example
Starting 2026-11-01 with 6 weeks to the deadline gives 2026-12-13, a Sunday. A start of 2026-11-01 and a deadline of 2027-01-15 is 75 calendar days, 10.71 weeks and 2 whole calendar months apart.
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Frequently asked questions
Does the start date count as day one?
No. The start date is the point of departure, so counting 7 days from Monday lands on the following Monday, the same as adding a week.
Why does a one month deadline land on a different day number?
Calendar months have different lengths. Adding one month to January 31 clamps to February 28 or 29, and adding one month to March 31 gives April 30.
Should the deadline be a business day?
This calculator counts calendar days, so the result can fall on a weekend. Move it to the next working day if your contract counts business days instead.
Can I measure backwards from a deadline?
Yes. Choose the time to the deadline mode with the deadline as the start date, or use the before option when adding a lead time to a fixed date.