The following formula calculates the number of minutes based on the fields you have available:
=IIf(IsNull([FromDate]),0,IIf(IsNull([EndDate]),0,(DateDiff("n",Format([FromDate] & " " & [FromTime],"dd/mm/yyyy hh:nn:ss"),Format([EndDate] & " " & [EndTime],"dd/mm/yyyy hh:nn:ss")))/60))
It also...