Sounds like you have a really huge number on your hands then, or you have some non-numeric data in that column. I'd usually try to break the problem down into chunks here. I'd start by doing:

[font="Courier New"]SELECT

   lastlogon

FROM

   TABLE

WHERE

   ISNUMERIC(lastlogon) <> 1[/font]

To see if I have any non-numeric characters although this doesn't always work as '+', '-', and '.' will return 1.