About this calculator

A date calculator that shows its rules instead of hiding them.

It is built and maintained by Marcio. Marcio builds small, single-purpose web tools that run entirely in your browser. This one exists because the same two questions keep appearing in travel and programming forums: 'what is the last day of my 90-day stay?' and 'what is January 31 plus one month?' Both have exact answers, but only once you know which counting rule applies.

Most date calculators give one number and leave you to guess how it was produced. This one does the arithmetic on whole calendar days (so daylight saving never shifts a result), applies the end-of-month rule used by spreadsheets and most contracts when adding months, and shows the "start date is day 1" answer alongside the standard one. Everything runs in your browser; no dates are sent anywhere.

The calculator does not know about public holidays, which vary by country and region, and it cannot tell you which counting rule a particular law, visa or contract uses. For anything with legal or financial consequences, confirm the date against the official source.

Spotted a wrong result or have a suggestion? Please use the contact page.