returns the value of time in seconds since 0 hours, 0 minutes,
0 seconds, January 1, 1970, Coordinated Universal Time.
If an error occurs,
returns the value
Po Vt time_t Pc Ns -1 .
The return value is also stored in
function may fail for any of the reasons described in
on failure; thus, it is impossible for an application to distinguish
the valid time value -1 (representing the last UTC second of 1969)
from the error return value.
Systems conforming to earlier versions of the C and
standards (including older versions of
did not set
in the error case.