Subject Subtract two TimeStamp-Fields
I have two fields in a table
Start TimeStamp (for example 19.02.2001 8:00)
End TimeStamp (for example 20.02.2001 9:00)
and I want the duration between these two fields. I tried something like

cast ((End-Start) as Numeric(9,3))
to get the seconds, that has gone by between these two points. But I get

an conversion error. Can anybody help me.