Since my recent post about converting between unix time and date, it has come to my attention that Unix epochs are in seconds while Java epochs are in milliseconds. That means that you need to scale the epoch values by a factor of 1000 to convert between Unix and Java format.
So, date -u -d '2011-04-26 15:00:00' +%s gives 1303830000 (Unix epoch) which becomes 1303830000000 (Java epoch)
Converting the other way, the Java epoch 1306934061475 is 1306934061.475 in Unix format. date -u -d @1306934061.475 gives Wed Jun 1 13:14:21 UTC 2011 (assuming a UTC epoch).