Accouting time duratin calculation

[Miha]

Hi,

how does freeradius calculate duration time? My NAS sends start and stop 
packet. In mysql table I can see accstarttime and accstoptime. Is the 
duration time automaticly created from start time and stop time or 
should NAS also send duration time?

In other words is it possbile that freeradius automaticly calculate 
duration from start and stop packet:)?

tnx!

miha