What units of time does "top" use?

ps and top display CPU time used, not clock time since the process started. One way to check when the process started is use the following command. The PID file creation date is when the process started:

ls -ld /proc/pid

So for process 2303 it would be:

ls -ld /proc/2303

minutes:seconds.hundredths

Searching for “TIME+” or for “seconds” gives the answer, kind of (I wouldn't call the man page clear).

This format is inherited from BSD, you also get it with ps u or ps l under Linux.