Date Add Calculator

How to use this calculator#

1. 1
   Set the start date and the directionAdd moves forward, subtract moves back. Every unit you enter is applied in the chosen direction, so there is no need to type negative numbers.
2. 2
   Use days for anything defined in daysNet 30, 90-day return windows, 14-day cooling-off periods and 180-day visa limits are all day counts in their governing text. Entering ‘3 months’ instead of ‘90 days’ can shift the answer by two days.
3. 3
   Use months and years for anniversariesRenewals, notice periods and subscription terms are calendar-based. A one-year term starting 10 May 2027 ends 10 May 2028, which is 366 days because 2028 is a leap year.
4. 4
   Check the weekday of the resultA deadline landing on a Saturday or a public holiday frequently moves to the next business day under the terms of the agreement — occasionally to the previous one. Either way, the calendar date alone is not the whole answer.

The formula#

The order the units are applied#

Years are applied first, then months, then weeks and days last. Order matters more than people expect: starting from 31 January, adding one month then 30 days is not the same as adding 30 days then one month. Applying the coarse units before the fine ones matches how contracts and statutes describe periods such as ‘three months and ten days from signing’.

Days and weeks are pure arithmetic — a week is always exactly 7 days, so 6 weeks is 42 days regardless of which months the span crosses. Months and years are calendar units, so their length depends on where you start. Adding 1 year to 10 May 2027 lands on 10 May 2028, a jump of 366 days because 2028 is a leap year.

Month-end rollover explained#

Adding one month to 31 January is ambiguous because February has no 31st. This calculator clamps to the last valid day of the target month, returning 28 February (or 29 in a leap year) rather than spilling forward into 2 or 3 March. Clamping is the convention used by most spreadsheet EDATE functions and by lenders setting payment dates.

The same rule applies in reverse. Subtracting one month from 31 March gives 28 February, and adding one month to 31 August gives 30 September. Be aware the operation is not always reversible: adding a month to 31 January and then subtracting a month returns 28 January, not the original 31st.

Common uses for date arithmetic#

Net-30 and net-60 invoice terms, 90-day return windows, 14-day cancellation periods and 180-day visa limits are all day-based, so use the days field for these. A 90-day window opened on 15 October 2027 closes on 13 January 2028. Contract renewals, subscription anniversaries and notice periods are usually month or year based instead.

The weekday of the result is worth checking before you commit to it. A deadline that lands on a Saturday or a public holiday often moves to the next business day under the terms of the agreement, which can quietly buy you an extra day or two — or cost you one, if the rule moves the date backwards instead.

Worked examples#

Reference tables#

Common mistakes#

• Treating 30 days as one monthOnly April, June, September and November have 30 days. A 30-day term from 31 January 2027 ends on 2 March, whereas one calendar month ends on 28 February — a four-day gap on a payment deadline, and the reason Net 30 is written in days rather than months.
• Applying the units in the wrong orderFrom 31 January, adding one month then 30 days gives 30 March; adding 30 days then one month gives 2 April. Both are defensible readings of ‘a month and thirty days’, which is why the order has to be stated and why this tool applies years and months before weeks and days.
• Assuming date arithmetic is reversibleAdding a month and then subtracting one does not always return the start date. Anywhere a schedule is generated by repeatedly adding a month — loan payments, subscription renewals — always generate from the original date rather than iterating from the last result, or a 31st will drift to a 28th and stay there.
• Ignoring the weekday the result lands onA deadline falling on a weekend or public holiday usually moves under the agreement's own rules, and different agreements move it in different directions. Checking the weekday is what turns a calculated date into a usable one.

Frequently asked questions#

Key terms#

Clamping

Capping the resulting day number at the last valid day of the target month, so 31 January plus a month becomes 28 or 29 February rather than spilling into March.

EDATE

The spreadsheet function that adds whole months to a date using the same clamping rule this calculator uses.

End-of-month rule

An alternative convention, common in finance, where a date falling on the last day of a month always maps to the last day of the target month.

Anniversary date

The same day and month in a later year — the basis for renewals, notice periods and annual terms.

Net terms

Payment windows expressed in days from the invoice date: Net 30, Net 60, Net 90. Always day counts, never calendar months.

Calendar unit vs elapsed unit

Years and months are calendar units whose length depends on where you start. Weeks and days are elapsed units of fixed size.

Sources#