I've recently been looking at reporting in OpenNMS and it seems that some of the reports require date ranges to be specified in unix time. Now, I don't know about you, but my mental ability to translate "April 26, 2011 15:00 UTC" to 1303830000 is sadly lacking. So, here are some useful commands to convert from unix time to date and vice versa:

Date to Unix time:

# current timestamp
date +%s

# specific timestamp
date -d '2011-04-26 15:00:00' +%s

# same thing but UTC
date -u +%s
date -u -d '2011-04-26 15:00:00' +%s

Unix time to date:

date -d @1303826400

# same thing but UTC
date -u -d @1303826400