Is 90 days the same as three months?

No. Three consecutive calendar months contain between 89 and 92 days, so the two periods usually end on different dates. February, March and April total only 89 days in a common year, while July, August and September total 92.

What happens when a monthly period starts on the 31st?

It ends on the last day of the target month if that month has no 31st. One month from 31 January ends on 28 or 29 February. This is known as the corresponding date rule, and it is not reversible: adding then subtracting a month can change the date.

Does the first day count when counting a notice period?

Usually not. The common convention is to exclude the day the period starts and include the final day, so a 30-day notice given on the 1st expires at the end of the 31st. Contracts using clear days exclude both the first and the last day.

What does net 30 mean on an invoice?

Payment is due 30 calendar days after the invoice date. Net 60 and net 90 work the same way with longer terms. Where terms are written as net 30 EOM, the count starts at the end of the invoice month instead, giving every invoice from that month a single due date.

90 Days From Today: Date Math

Someone asks for a decision within 90 days and someone else diarises it for three months' time. Those are not the same date, and depending on where in the year you start they can differ by three days in either direction. Working out 90 days from today looks trivial until the period crosses February, a month end, or a contract clause that defines counting differently from how you would count it yourself.

This article explains why days and months are different units, where the end-of-month edge cases bite, and how commercial agreements usually resolve the ambiguity. The this month page shows how much of the current month is left, which is the first thing you need when counting forwards.

Why 90 Days From Today Is Not Three Months

Because months are not 30 days long. Three consecutive calendar months contain between 89 and 92 days depending on which three they are, so a 90-day period and a three-month period will usually land on different dates.

February, March and April total 89 days in a common year — the shortest three-month span there is. July, August and September total 92, the longest. A period of 90 days therefore overshoots a three-month term in spring and falls short of it in late summer. Two people applying two definitions to the same agreement can be three days apart in good faith, which is enough to miss a filing deadline. The month lengths driving all of this are listed in how many days in each month.

The same applies at the shorter terms. Thirty days is not one month, since only four months have exactly 30 days, and 60 days is not two months in any pair except a 30-day month followed by another. Only the 90-day case gets close often enough that people stop noticing the difference, which is precisely why it causes trouble.

The End-of-Month Edge Cases

The awkward cases all involve a start date near the end of a long month. Add one month to 31 January and there is no 31 February to land on; add one month to 31 August and 31 September does not exist either. Something has to give, and different systems give differently.

The usual resolution is the corresponding date rule: a period of months ends on the day before the same numbered day in the later month, and where that day does not exist, it ends on the last day of that month instead. So one month from 31 January ends at the end of February, on the 28th or the 29th. That rounding is not reversible — going forward one month and back one month can return you to a different date than you started on, which is a genuine source of bugs in scheduling software. Whether February supplies a 28th or a 29th depends on the leap rule set out in February and leap years.

How Contracts Define 30, 60 and 90 Days

Well-drafted agreements say exactly what they mean, and the common formulations differ in ways worth recognising:

Calendar days: every day counts, including weekends and public holidays. This is the default reading of "90 days" unless stated otherwise.
Business days: weekends and holidays are skipped, so a 30-business-day period runs roughly six weeks and depends on which holidays fall inside it.
Clear days: both the first and last days are excluded, adding a day compared with ordinary counting. Common in court rules and notice periods.
Net 30, 60 or 90: payment terms counted from the invoice date rather than from delivery or acceptance, which is why the invoice date is negotiated as carefully as the term itself.
End-of-month terms: written as "net 30 EOM" or similar, the count begins at the end of the invoice month, so every invoice in a month shares a due date.

The general legal convention is that the first day is excluded and the last day included, so a 30-day notice served on the 1st expires at the end of the 31st. If the final day falls on a weekend or holiday, many contracts apply a business-day convention that moves it to the next working day, unless doing so would push it into the following month, in which case it moves back.

Day-Count Conventions in Finance

Financial markets sidestep the whole problem by declaring the month lengths to be something they are not. The 30/360 convention treats every month as 30 days and every year as 360, which makes interest for any whole month identical and coupon arithmetic trivial.

Other conventions keep reality and adjust the denominator: actual/365 counts real days over a 365-day year, and actual/360 counts real days over a 360-day year, which quietly produces slightly more interest for the lender. None of these are wrong; they are agreed fictions, and the only error is applying one convention while your counterparty applies another. The same instinct explains why so many companies report on 13-week quarters rather than calendar months, described in fiscal quarters explained.

How Do You Count 90 Days From Today Correctly?

Count real calendar days, not months, and check the rule you are working under before you start. Work forward month by month, subtracting each month's length from the total until the remainder is smaller than the next month.

Fix the start date and the rule. Decide whether the first day counts, and whether the days are calendar or business days.
Use up the current month. Subtract the days remaining in it from your total. The days in month page gives the length if you are unsure.
Subtract whole months. Take off the full length of each following month while the remainder allows it, remembering February's two possible values.
Land on the day. The remainder is the day of the month you finish on. Check next month if the period ends just over the boundary.
Adjust for weekends. Apply the business-day convention only at the end, never during the count.

Long periods often cross a quarter boundary, which can matter for reporting even when it does not change the date; the quarter page shows which quarter the finish date lands in.

One habit is worth adopting whatever the rule: write the resulting date down rather than the phrase. A diary entry saying "90 days from today" is only meaningful on the day it was written, and it forces whoever reads it later to redo the arithmetic with a start date they may have to guess. Recording the calculated date, and the basis it was calculated on, removes both problems at once.

Conclusion

Counting 90 days from today gives a different answer from counting three months, because three consecutive months run anywhere from 89 to 92 days. End-of-month start dates need the corresponding date rule, February needs the leap year check, and contracts need reading closely enough to know whether the days are calendar days, business days or clear days. Count in days when the agreement says days, and in months only when it says months. To start the count from where you are now, open this month or the other calendar tools on months.now.

30, 60 and 90 Day Date Math: Working Out 90 Days From Today