timestamp in radpostauth and radacct

Hey, To include information about the NAS in a listing of the radpostauth (which doesn't have an entry regarding the nas from which the request originated from) I am joining the tables based on the radacct.acctstarttime = radpostauth.date which works "part" of the time. Meaning, for most of my tests it works just fine but I've seen logs where the timestamp in both of these tables differ (for the same request ofcourse). I was hoping someone can clear this issue, what's a normal behavior here? are the timestamp supposed to be the same/different and why. Regards, Liran.