Add and Subtract Time

A small tool for calculating with time data. Insert two periods of time and choose, if you want them to add or to subtract. The transitions between the time units are calculated correct.

Example: 16 hours, 30 minutes plus 12 hours, 45 minutes adds up to one day, 5 hours and 15 minutes.

One minute has 60 seconds, one hour has 60 minutes or 3600 seconds, one day has 24 hours or 1440 minutes or 86400 seconds. An average year has a length of 365.24 days.

On these pages you will find a wide variety of calculators for times and dates, including some very special ones. The mathematical peculiarity about times is that they are written in the decimal system (tens system), but not calculated. Sixty seconds is a minute, 60 minutes is an hour and 24 hours is a day. The division into sixty steps, the sexagesimal system, dates back to the ancient Persians, who adopted this from the even older Babylonians. Dividing the day into 24 hours, on the other hand, comes from ancient Egypt. Historically this is interesting, but of course it does not simplify the practical calculation. The large number of individual calculators results from these time indications, which are difficult to calculate, and the possible applications.

Astronomical background: the only common time specifications that can be derived from real conditions, i.e. are given, are the day and the year (and originally the month). A day, understood as a full day from midnight to midnight, is the time between two meridian crossings of the sun, the time it takes for the sun to return to the same point. This is caused by the earth's rotation around itself, but is not equal to the rotation period, since the earth is also rotating around the sun at the same time. The period of rotation of the earth around itself is about four minutes shorter. After all, the earth needs about 365 1/4 days on its orbit around the sun, which makes the regulation with leap years about every 4 years necessary. The month is derived from the moon, but the months are slightly longer than the orbital period of the moon around the earth.