Add / Subtract Days

Ninety days from 1 January

Add 90 days to 1 January 2025 and you arrive at 1 April 2025, a Tuesday.

Day arithmetic is the one date operation with no ambiguity at all. Unlike months and years, whose lengths vary, a day is a day — so adding 90 days is simply counting forward 90 times, leap days included, with no conventions to choose and no clamping to worry about.

That reliability is exactly why so many legal, financial and medical deadlines are expressed in days rather than months. "Within 90 days" has precisely one meaning; "within three months" needs a rule for what happens when you start on the 31st.

How the count works

Result = start date + (number of days)

Working the example: January has 31 days, so 1 January + 31 = 1 February. February 2025 has 28 days (2025 is not a leap year), so +28 = 1 March, at day 59. March has 31 days, and 90 − 59 = 31, so we land exactly on 1 April.

Had the same 90 days started from 1 January 2024, February would have contributed 29 days and the answer would be 31 March — one day earlier in the calendar, from an identical count. Leap years shift every span that crosses them.

Deadlines that are counted in days

Cheque validity — three months in India, but many instruments and mandates specify a day count instead.

Income tax and compliance windows — advance tax, TDS deposit deadlines, GST return periods and appeal windows are frequently expressed in days from a triggering event.

Notice periods — usually 30, 60 or 90 days in Indian employment contracts, counted in calendar days unless the contract says otherwise.

Return and warranty windows — 7, 15, 30 days from delivery.

Medical and vaccination schedules, where intervals are specified in days precisely because they must not vary.

Rent agreements and lock-in periods, and the notice required to terminate them.

The inclusive-versus-exclusive question

This is the most consequential detail and the one most often left unstated. If a contract says "within 30 days of receipt," does the day of receipt count as day one, or does counting begin the following day?

The general convention in Indian law, following the General Clauses Act, is to exclude the first day and include the last. A 30-day period from 1 January therefore ends on 31 January, not 30 January.

But contracts can and do specify otherwise, and commercial practice varies. When a single day matters — and for a limitation period or an appeal window it can matter absolutely — read the governing clause rather than assuming.

Reading the result

The day of the week is frequently what determines the effective deadline. Where a period expires on a Sunday or a gazetted holiday, most Indian statutes allow the act to be done on the next working day. Banking and settlement conventions sometimes move backwards instead.

Remember also that this counts calendar days. Ninety working days is roughly 126 calendar days once weekends are excluded, and more once public holidays are counted.

Common mistakes

Getting the first day wrong. Excluding or including the start date changes every deadline by one.

Forgetting leap years. Any span crossing 29 February shifts by a day.

Confusing 90 days with three months. From 1 January they are the same; from 1 December they are not.

Assuming working days. Unless stated, a day count means calendar days.

Missing the holiday extension. A deadline falling on a non-working day usually moves, but you need to know which way.