"Unix time, or POSIX time, is a system for describing points in time, defined as the number of seconds elapsed since midnight proleptic Coordinated Universal Time (UTC) of January 1, 1970, not counting leap seconds." (http://en.wikipedia.org/wiki/Unix_time)
If you have a date object in java, you just call the date.GetTime() function to return that value.
.Net does not work well with this and I just found myself coding happily along until i hit this and realized i had to write this handy little C# function i now share with the world:
DateTime ConvertEpochToDate(long epochMS)
DateTime disco = new System.DateTime(1970, 01, 01, 00, 00, 00, 00);
You can see, we add the milliseconds i have to a new datetime object that has been initialized to the unix epoch