The date command

All values in "seconds" is the number of seconds from Epoch, Jan 1, 1970.

1. Current time in seconds.
   date +%s
1220815239
2. Convert 1220815239 seconds to a human readable date.
   date -d @1220815239
Sun Sep 7 15:20:39 EDT 2008
3. Convert 1220815239687 milliseconds to a human readable date.
   date -d @$[1220815239687/1000]
Sun Sep 7 15:20:39 EDT 2008

Use these commands when you need to convert on the fly. Be mindful that some Unix timestamps may be presented in UTC (GMT). If working in GMT, append -u to date.

date -d @$[1220815239687/1000] -u
Sun Sep 7 19:20:39 UTC 2008