This:
date +%s
will return the number of seconds since the epoch.
This:
date +%s%N
returns the seconds and current nanoseconds.
This:
date +%s%3N
returns the seconds and current milliseconds.
If you want to convert back use following command
date -d @1501513250
or you can format timestamp via formater
date -d @1501513250 +'%Y-%m-%d %H:%M:%S'