How date arithmetic works
- Days and weeks are exact: one week is always 7 days.
- Months move to the same day number in the target month. If that day does not exist, the result is the last day of that month.
- Years are 12 months, so February 29 plus one year becomes February 28.
Common uses
- Payment and contract deadlines (net 30, 60 or 90 days).
- Notice periods, probation periods and warranty expiry dates.
- Medication schedules and follow-up appointments.
- Project planning and countdowns.
How to use the calculator
- Choose a start date (it defaults to today).
- Choose whether to add or subtract time.
- Enter any combination of years, months, weeks and days and read the resulting date and weekday.
How months and leap years are handled
Adding days and weeks is exact: the calculator counts calendar days. Adding months is trickier because months have different lengths. The calculator keeps the same day of the month when possible and moves to the last day of the month when that day doesn’t exist — so one month after January 31 is February 28 (or 29 in a leap year), and one month after March 31 is April 30. Years are applied first, then months, then weeks and days.
Leap years have 366 days, with February 29. A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400 — so 2000 was a leap year and 2100 will not be. Adding one year to February 29, 2028 gives February 28, 2029.
Common date calculations
- Deadlines: “within 30 days of receipt”, notice periods, return windows and payment terms (net 30, net 60).
- Contracts and leases: end dates 6, 12 or 18 months from the start.
- Health: medication schedules, follow-up appointments, and quarantine or recovery periods.
- Planning: countdown to an event, or the date 100 days from now.
Calendar days vs. business days
Legal and business deadlines sometimes count only working days, and rules differ on whether the start day is included. In many legal systems the day of the triggering event is not counted, and a deadline that falls on a weekend or public holiday moves to the next working day. If weekends and holidays should be excluded, use the business days calculator, and always check the rule that applies to your situation.
Useful reference
- 1 week = 7 days · 4 weeks = 28 days · 13 weeks ≈ 3 months
- 1 year = 52 weeks and 1 day (2 days in a leap year), which is why your birthday moves one weekday later each year
- The average month is 30.44 days (365.25 ÷ 12)