The duration between two UTC timestamps can be computed

[ThatJoeMoore]

Due to the unpredictable nature of leap seconds, an interval that includes any date in the future cannot be guaranteed to be accurate. At any time, the IERS can declare a leap second, which must occur in the last minute of the month (June and December are preferred). In practice, the IERS gives about six months advance notice before a leap second, but is not required to do so. Because a leap second could be added at the end of any month, only intervals that do not span the last day of a month can be guaranteed to be accurate to the second.

In order to compute durations between timestamps in the past, a table of all leap seconds must be consulted.