Add or minus days from any date

How date addition work

Year and month add first, then week and day. When you add month, e get respect for how many day dem month get. For example, if you add 1 month to January 31, you go get February 28, or February 29 if na leap year, not March 3.

E useful for calculate deadline, expiry date, contract end date, and countdown for any duration.

Quick reference

Month-end edge case

Month addition na the tricky part of date arithmetic because month no get the same number of day. The standard rule wey this calculator use — and wey most programming language and legal convention use — na to land on the last valid day of the target month when the source day no exist there.

Example:

• January 31 + 1 month = February 28 (or 29 if na leap year)
• January 31 + 2 months = March 31
• August 31 + 1 month = September 30
• October 31 + 4 months = February 28 (or 29 if na leap year)

E get name "end-of-month clamping." E prevent say you no go spill into the next month (March 3 instead of February 28), wey go change the month count and go break the meaning of "+1 month."

Calendar day versus business day

Add calendar day na straightforward: 30 day from any date na exactly 30 day later, even if weekend or public holiday fall inside. Add business day na more complicated and e depend on which public holiday dem observe for the place — e differ by country, state, and even industry.

For most legal and contract matter, "day" mean calendar day except if dem say something different. "Business day" or "working day" mean day wey no include weekend and named public holiday. The calculator add calendar day; if you need business day, add am yourself or count weekend in your figure (rough 20–22 business day per 30 calendar day).

Practical application

Contract and legal deadline

Contract duration almost always get express in month or year: 12-month lease, 3-year service agreement, 90-day right of withdrawal. You need calculate the end date correct correct, because "one year from March 15" na March 15 of the next year, not 365 day later (wey go be March 14 if no leap year, or March 15 if leap year).

Trial period and subscription

Free trial, return window, and subscription billing cycle usually get express in day: 14-day free trial, 30-day return policy, 60-day money-back guarantee. Dem na calendar day count from the start date. Add the exact number of day go give you the precise expiry date.

Probation and notice period

Employment probation period usually get state in month (3-month probation, 6-month probation). Notice period get state in week or month. Add dem to a start date go give you the exact end date of the probation or the earliest departure date after you serve your notice.

Project planning and scheduling

Add duration to milestone date go give you a forward project schedule. Work backward from a fixed deadline — subtract duration — show you the latest possible start date for each phase. Both side use the same date arithmetic.

Warranty and guarantee expiry

Product warranty, insurance policy, and guarantee period all get tie to date. A 2-year warranty wey start on a purchase date go expire exactly 2 year later, wey no always be 730 or 731 day later because of leap year — e na the same day and month two year after.

How to calculate a date by hand

For add day: convert to a day number, add, convert back. For add month: add the month count to the month number, carry over to year if needed, clamp the day to the last day of the month wey you go get. For add year: add direct to the year, then apply the same leap-year clamp if the original date na February 29.

The calculator do all of this automatic — include leap year detection and month-end clamping — so you fit enter any start date and any combination of year, month, week, and day and get reliable result.