current-time-string is a built-in function in `C source code'.

(current-time-string &optional SPECIFIED-TIME)

Return the current local time, as a human-readable string.
Programs can use this function to decode a time,
since the number of columns in each field is fixed
if the year is in the range 1000-9999.
The format is `Sun Sep 16 01:03:52 1973'.
However, see also the functions `decode-time' and `format-time-string'
which provide a much more powerful and general facility.

If SPECIFIED-TIME is given, it is a time to format instead of the
current time. The argument should have the form (HIGH LOW . IGNORED).
Thus, you can use times obtained from `current-time' and from
`file-attributes'. SPECIFIED-TIME can also have the form (HIGH . LOW),
but this is considered obsolete.