Date & Time Calculators
5 free date & time calculators, each explaining its formula.
Ages, durations, deadlines, working days and time zones — the calculations that are deceptively easy to get wrong by one day.
Age Calculator
Exact age is not a subtraction. Months have different lengths, so the answer depends on the order you count in.…
Date & TimeDate Difference Calculator
Two dates in, every useful measure of the gap out — including the inclusive and exclusive counts, which differ by one and cause en…
Date & TimeWorking Days Calculator
Contracts, notice periods and delivery estimates are almost always counted in working days, which is a different number from calen…
Date & TimeTime Duration Calculator
Built for timesheets: it handles overnight shifts, subtracts breaks, and gives the decimal hours that payroll systems expect.…
Date & TimeAdd or Subtract Days Calculator
Includes the end-of-month rule, which is where date arithmetic quietly goes wrong.…
About this section
Date arithmetic looks trivial and is not. Months have different lengths, years occasionally have an extra day, time zones shift twice a year in many places, and there are two entirely legitimate ways to count the days between two dates that differ by one.
That last point causes more practical problems than any other. From the first of the month to the third is two days later and covers three days. A contract saying “within fourteen days” and one saying “fourteen clear days” produce different deadlines. A hotel stay of three nights spans four calendar days. None of these is ambiguous once stated; all of them are ambiguous when assumed.
Adding months is worse, because there is no correct answer to what falls one month after the thirty-first of January. The convention almost all software uses, and the one applied here, is to clamp to the last day of the shorter month. That convention is not reversible, which means chaining month additions accumulates drift — a detail that matters for anyone calculating a schedule of recurring dates.
The tools below work in coordinated universal time throughout, so a day is always exactly twenty-four hours and daylight saving transitions cannot introduce a spurious extra day. Each page states which convention it uses and why.