日付計算

The date calculator does two jobs: it measures the time between two dates, and it adds or subtracts a period from a date. It shows the result as a year, month and day breakdown, a total number of days and weeks, and the weekday of the resulting date.

Everything is calculated in your browser, so the dates you enter never leave your device.

Counting days sounds simple but has a catch: do you count both endpoints or the nights between them? By default the calculator counts the nights, so the start day is not double-counted, with an option to include both the first and last day when you need an inclusive count.

Adding months or years keeps the same day of the month where possible. If the target month is shorter, for example adding a month to 31 January, the date rolls forward following standard calendar behaviour, so the result is always a real date.

All calculations use real calendar dates, so leap years and the differing lengths of months are handled correctly rather than approximated.

It is handy for everyday planning: project deadlines, payment and due dates, notice and cancellation periods, ages and anniversaries, and working out which weekday a date falls on.