Returns: A
sys_time that is at least as fine as
seconds,
and will be finer if the argument
tp has finer precision
. This
sys_time is the UTC equivalent of
tp
according to the rules of this
time_zone. If the conversion from
tp to a
sys_time is ambiguous,
returns the earlier
sys_time if
z == choose::earliest, and
returns the later
sys_time if
z == choose::latest. If the
tp represents a non-existent time between two UTC
time_points,
then the two UTC
time_points will be the same,
and that UTC
time_point will be returned
.