Add or subtract days, weeks, calendar months or years. See the resulting date, weekday and elapsed days, with month-end and leap-year handling.
Formula and sources updated · Data sources · 1View algorithm, constants and default inputs (JSON)
Use strict Gregorian dates from year 0001 to 9999. A combined offset adds years and months together, clamps the day once to the target month end, then applies weeks and days.
Recomputed from this page's implementation at build time · checked 2026-10-29
Date arithmetic breaks wherever a calendar runs out of days. This page fixes what happens at month ends and leap days, explains why a subtraction can produce a negative day count, and hands the other half of the question — the gap between two dates — to a tool that exposes its own switch.
| 7 | 30 | 90 | 365 | |
|---|---|---|---|---|
| days | 7 | 30 | 90 | 365 |
| months | 212 | 913 | 2,739 | 11,109 |
| years | 2,557 | 10,958 | 32,872 | 133,313 |
One number, three units. Ninety means ninety days, or seven and a half years when read as months, or ninety years as years - a spread of three orders of magnitude from a single dropdown. The unit field, not the number field, is where deadline arithmetic actually fails.
Nothing is clamped here, so this card is the baseline: the day count and the calendar agree, and the weekday is a property of the result rather than of the input.
February has no 31st, so the result falls back to the last valid day. The competing convention is to overflow into March, which spreadsheet date functions commonly do; both are defensible, and they return different dates from identical input.
The target year has no 29th of February, so the result clamps to the 28th. Notice that the day count reflects the clamp: the same nominal jump of one year covers a slightly different number of days depending on which year you leave from.
The negative number is the answer, not a fault: it records that the result precedes the start date. Adding and subtracting the same amount produce counts of equal size and opposite sign.
Zero is a legal amount and returns the input unchanged. The day count is zero and the weekday is simply whatever that date already was — useful when you are checking that the field wiring is correct.
Two dates describe a span, but how many days it contains depends on whether the last day counts. This page does not take two dates at all, so the convention question belongs to the companion tool, which exposes it as a switch.
Exactly one more day, and one more weekday in this case. Neither count is wrong; a page that hides the switch is quietly choosing for you, and the choice shows up in payroll, deadlines and notice periods.
inputError
A date that does not exist
The text matches the date pattern but no 30th of February exists. Rolling it forward to March 2 would answer a question the user did not ask, so the whole calculation stops.
inputError
An amount beyond the accepted range
The limit is on the amount, not on the resulting year. Large values are still accepted well inside the cap, so the refusal marks a boundary of the input rather than of the calendar.
inputError
A negative amount
Going backwards is expressed by the operation selector, not by the sign of the amount. Accepting both would create two ways to say the same thing and therefore two chances to disagree.
calculator.netchecked 2026-10-22
Why they built it that wayAdding and subtracting lives on its own page, apart from the difference-between-dates page, because the two are typed as different questions. The cost is that neither page can show you both answers at once.
If your task is counting working days, this page cannot do it and will not pretend to. The weekday is reported, but weekends are not skipped — the companion tool is the honest route for that question.
Set the start date to January 31 of any year, add one month, and confirm the result is the last day of February rather than a March date. Then subtract 30 days from today and check that the day count is negative.
Recounted by adding up month lengths by hand, and cross-checked the same pair of dates on the date-difference page · impl bcf16a9c
Use strict Gregorian dates from year 0001 to 9999. A combined offset adds years and months together, clamps the day once to the target month end, then applies weeks and days. Subtraction reverses the offset. Complete months follow that same clamp rule. The reference date changes only the result-distance note, not the result date. No holidays or time zones are applied.
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
R = addDays(clampMonth(S, ±(12Y + M)), ±(7W + D))
The reference date changes only the result-distance note, not the result date. No holidays or time zones are applied.
Use strict Gregorian dates from year 0001 to 9999. A combined offset adds years and months together, clamps the day once to the target month end, then applies weeks and days. Subtraction reverses the offset. Complete months follow that same clamp rule. The reference date changes only the result-distance note, not the result date. No holidays or time zones are applied.