Get the current Datetime:

~> date
Thu Aug 23 00:45:05 CEST 2012

Get universal Timestamp:

~> date +%s -u
1345675505

Set universal Datetime by Timestamp:

~> date -u -s "1970-01-01 1345675505 sec"
Wed Aug 22 22:45:05 UTC 2012

Check the new Datetime:

~> date
Thu Aug 23 00:45:05 CEST 2012