These sites might help you:

http://www.sqlteam.com/article/datediff-function-demystified

http://www.eggheadcafe.com/community/aspnet/13/10010433/how-to-calculate-last-log.aspx

or might be this one will help:

http://www.eggheadcafe.com/software/aspnet/30238367/datediff-problem.aspx