When you store the login time in teh Session, are you converting it to a string? If so, then that is your problem - the default ToString implementation does not return the millseconds. Instead, store it as a DateTime directly, and cast it back when you retrieve it. That way, it does not lose any information.
See MSDN:
http://msdn.microsoft.com/en-us/library/ms178581.aspx[
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