I have a log table like this:

username productID purchasetime
---------------------------------------------------
user1 12 01/02/1999
user1 12 03/05/2000
user2 23 02/03/2000
user2 31 02/04/2000
user2 23 02/04/2000
....

I want to select users that purchased product later than a predefined time, say 01/01/1998, but if a user purchase a same product at different dates, then only the latest one will be selected. How to write the query? Thanks a lot.