About DateAndUnits

DateAndUnits is a set of free date and time calculators. There is no account, no paywall, and no limit on how often you use them.

The site exists because most date arithmetic is harder than it looks and the tools that handle it often round off the awkward parts. Every calculator here is built around the edge cases rather than in spite of them.

The rules these calculators follow

Leap years use the complete rule, not the shortcut. A year divisible by 4 is a leap year, except years divisible by 100, unless they are also divisible by 400. That means 2000 was a leap year and 2100 will not be. Plenty of date tools apply only the divisible-by-4 part, which produces a wrong answer for February 2100 onward.

Month arithmetic clamps to the last valid day of the target month. Adding one month to January 31 gives February 28, or February 29 in a leap year, because February 31 does not exist. This matches how contracts, leases, and billing systems read the same instruction. A side effect is that the operation is not reversible: go forward a month from January 31 and back again, and you land on January 28.

Months are never treated as a fixed 30 days. Three months and 90 days are different spans, and the gap between them varies depending on which months you cross. Where a distinction matters, the calculators keep them separate rather than blurring them.

Clock calculations show the date whenever a result crosses midnight in either direction. A bare time like 5 AM is ambiguous; 5 AM on the following day is not.

How the results are checked

Each calculator is tested against the cases that most commonly break date tools. That includes February 29 arithmetic in both directions, month-end clamping, century years that are not leap years, spans that cross a year boundary, midnight rollover forwards and backwards, reversed date ranges, and zero-length spans.

The month and year functions are checked against the same clamping convention used in contract law, since that is the context most people are working in when they need the answer.

If you find a result that looks wrong, it is worth reporting. Date arithmetic has a lot of corners and an outside test case is genuinely useful.

What these calculators do not do

They count calendar days, not business days. Working-day counts depend on which country's public holidays apply, and those vary enough that a single generic answer would be misleading.

Clock calculations use your device's local timezone. They do not convert between timezones, and results that cross a daylight saving boundary can differ from wall-clock expectations by an hour.

Nothing here constitutes legal advice. Where a contract defines its own counting rule, such as whether the first day is included, that rule takes precedence over any general convention.

Privacy

Every calculation runs in your browser. The dates and times you enter are never sent to a server, never logged, and never stored. Closing the tab is enough to clear them.

That is partly a privacy decision and partly a speed one. Nothing has to travel anywhere, so results appear as you type.

Frequently asked questions

Who runs DateAndUnits?

DateAndUnits is an independent site. It is not affiliated with any calendar software vendor, and no calculator result is influenced by a commercial relationship.

Are the calculators free?

Yes. Every tool is free, with no account required and no usage limits.

Is my data collected?

No. All calculations run locally in your browser, so the dates and times you enter never leave your device.

How do I report a wrong result?

Use the contact page and include the exact inputs you used. Specific reproduction steps make a real difference in tracking down date arithmetic bugs.