What is January 31 plus one month?

It is the final valid day of February: February 28 in a common year or February 29 in a leap year.

Does adding one year to February 29 return March 1?

No. This calculator clamps the result to February 28 when the target year is not a leap year.

Can DST change the result?

No. The tool shifts calendar dates rather than adding 24-hour durations to local timestamps.

Does this skip weekends or public holidays?

No. Use the Working Days Calculator when weekends or a supplied holiday list must be excluded.

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Add or Subtract From a Date

Move a date by an exact number of days, weeks, months or years with predictable month-end handling.

Add or subtract

Month-end aware

No DST drift

Adds or subtracts whole calendar units from an ISO date, clamping invalid month-end results to the destination month’s final day.

Limitation: This tool performs calendar arithmetic, not legal deadline computation.

Jump to calculator · Sources & review

Free Calculators HubDate Calculator

Start dateOperationWhole amountUnit

Understanding Add or Subtract From a Date

How to Add or Subtract Dates

Choose a start date, operation, whole-number amount and unit. Days and weeks represent fixed counts of calendar dates. Months and years preserve the day number where possible and otherwise use the final valid day of the target month.

Select the date where the calculation begins.

Month-end rule

Adding one month to January 31 produces February 28, or February 29 in a leap year, because February has no 31st day.

How Calendar Date Arithmetic Works

This calculator answers a date-shifting question, not an interval-length question. For day and week operations, it advances or reverses by calendar-day indexes. For month and year operations, it changes the month or year first, then clamps the original day to the last day available in the destination month. That explicit rule prevents rollover surprises such as treating January 31 plus one month as a date in March.

Date Arithmetic Examples

Add a month at leap-year month end

Start and adjustment

startDate:2024-01-31

operation:add

amount:1

unit:months

Result date

2024-02-29

The day is clamped from 31 to February 2024’s last valid day.

Subtract a day across a month boundary

Start and adjustment

startDate:2025-03-01

operation:subtract

amount:1

unit:days

Result date

2025-02-28

A one-day calendar shift crosses into February.

Deadline caution

If a real deadline moves when it falls on a weekend or holiday, calculate the raw date here and verify the governing rule separately.

Frequently Asked Questions

The start date is day zero. Adding one day returns the next calendar date; adding zero days returns the start date.

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Date Addition and Subtraction Formula

The selected unit determines whether the operation uses day indexes or calendar fields.

Imperial:For imperial units: target month = start month ± amount; result day = min(start day, days in target month)

Formula

result day index = start day index ± amount × unit length

Days and weeks

result day index = start day index ± amount × unit length

Months and years

target month = start month ± amount; result day = min(start day, days in target month)

Scientific Background

Dates use the proleptic Gregorian calendar. They are date-only values: no local clock time, UTC offset or daylight-saving transition is added.

Sources & review

Date Calculator documents the references behind its formulas and assumptions. Always treat results as educational estimates, not personalized professional advice.

Last reviewed

July 21, 2026

Reference
rfc-editor.org
https://www.rfc-editor.org/rfc/rfc3339#section-5.6
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