Nearly all modern operating systems assume that 1 day = 24 × 60 × 60 = 86400 seconds in all cases.In UTC, however, about once every year or two there is an extra second, called a "leap second." The leap second is always added as the last second of the day, and always on December 31 or June 30.Leap seconds are introduced as needed into UTC so as to keep UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections applied.

object is modified so that it represents a point in time within the specified hour of the day, with the year, month, date, minute, and second the same as before, as interpreted in the local time zone.

object is modified so that it represents a point in time within the specified minute of the hour, with the year, month, date, hour, and second the same as before, as interpreted in the local time zone.

The distinction between UTC and UT is that UTC is based on an atomic clock and UT is based on astronomical observations, which for all practical purposes is an invisibly fine hair to split.

Because the earth's rotation is not uniform (it slows down and speeds up in complicated ways), UT does not always flow uniformly.

Note that this is slightly different from the interpretation of years less than 100 that is used in object is modified so that it represents a point in time within the specified day of the month, with the year, month, hour, minute, and second the same as before, as interpreted in the local time zone.

If the date was April 30, for example, and the date is set to 31, then it will be treated as if it were on May 1, because April has only 30 days.

An interesting source of further information is the U. Naval Observatory, particularly the Directorate of Time at: .

Determines the date and time based on the arguments.

For example, the last minute of the year 1995 was 61 seconds long, thanks to an added leap second.