If you're using Joda (which may be coming as jsr 310 in JDK 7, separate open source api until then) then there is a Seconds class with a secondsBetween method.
Here's the javadoc link: http://joda-time.sourceforge.net/api-release/org/joda/time/Seconds.html#secondsBetween(org.joda.time.ReadableInstant,%20org.joda.time.ReadableInstant)