Debug filenames are composed of
the current time measured in the number of seconds since the Unix Epoch (January 1 1970 00:00:00 GMT)
and a random number.

Why the random number?

I changed the format to
PHP Code:
$debug_logfile_path = DIR_FS_SQL_CACHE . '/myDEBUG-' . date("Ymd_G-i-s", time()) . '.log'; 
which I found easier to understand.

ps
The comments in the error_reporting files regarding the format are incorrect.