The type denoted by file_clock provides
precisely one of the following two sets of static member functions:
template<class Duration>
static sys_time<see below>
to_sys(const file_time<Duration>&);
template<class Duration>
static file_time<see below>
from_sys(const sys_time<Duration>&);
or:
template<class Duration>
static utc_time<see below>
to_utc(const file_time<Duration>&);
template<class Duration>
static file_time<see below>
from_utc(const utc_time<Duration>&);