I'll try to help, although I'm not exactly clear what the intent is. I'm not sure if you want the Employee with the greatest Timestamp or the greatest Timestamp itself.
If you want the Employee record with the greatest timestamp then you can order the result set by timestamp in descending order and get the first record:
Code:
.addOrder(Order.desc("timestamp"))
Another option would be to have a sql subquery, although this would not be as portable.
Another option would be to use a projection to get the max, but since I don't have too much experience with this I won't comment further.
Hope this helps!