About AgoCalc

A small set of date and time tools, built to answer one question each and to show how the answer was reached.

What this site is

AgoCalc is a collection of free date and time calculators. Each tool covers a single question, such as what date it was a given number of days ago, or how many days sit between two dates. There is no account, no paywall and nothing to install.

How the calculations run

Every calculator is plain JavaScript running in your browser. When you type a number or pick a date, the arithmetic happens on your own device and the result appears straight away. No dates are transmitted to a server, which means the tools keep working on a slow connection and the values you enter stay with you.

The conventions we use

Date arithmetic involves several judgement calls. These are the ones this site makes, applied consistently across every tool.

  • Local midnight. Dates are handled at local midnight rather than as timestamps, so results do not shift when your time zone crosses a day boundary.
  • Elapsed days. The gap between two dates counts complete days and excludes the start date, which is the standard used for interest, age and most statutory periods. Tools that offer an inclusive count label it clearly.
  • Month-end clamping. Subtracting one month from March 31 gives February 28, or February 29 in a leap year, rather than an invalid date. This follows ISO 8601 and matches how billing and contract systems behave.
  • Leap years. A year divisible by 4 is a leap year, except century years, which must also be divisible by 400. So 2000 was a leap year and 1900 was not.
  • Business days. Working-day tools skip Saturdays and Sundays. They do not exclude public holidays, because those differ by country and region. Where a deadline depends on holidays, check them separately.

Accuracy and limitations

The tools use the Gregorian calendar, which most countries adopted at different points between 1582 and 1927. Calculations spanning dates before a country made that switch may not match historical records for that country. For everyday use, and for any date in living memory, the results are exact.

These calculators are provided for general information. For legal deadlines, medical scheduling or financial settlement, confirm the result against the rules that govern your specific situation, since those often define day counting in their own terms.

Corrections

If a result looks wrong, it is worth reporting. Date arithmetic has a lot of edge cases and the fastest way to find the remaining ones is for someone to hit them.