Not TLB related, but you might be able to help. :)

I am pretty sure Net Monitor does not grab any information from your ISPs info collected about online time, etc.

It just sits there and looks for a connection and when a connection is made it starts the timer...
So if your explorer crashed for any reason you would need to drop the connection and reconnect or just wait until your connection drops and you reconnect inorder for the time online displayed by Net Monitor to be true.
(Net Monitor needs to be loaded at time of connection)

Hope that helps