The POSIX standard provides three functions for converting between integer
epoch values and 6-component "broken-down" time representations. localtime
and gmtime convert an epoch into the 6 components of seconds, minutes,
hours, day of month, month and year, in either local timezone or UTC. The
mktime function converts a local broken-down time into an epoch value.
However, POSIX does not provide a UTC version of this.
