Hi ,
Create 1 variable For Submission Date .
var_Sub_Date = if Isnull([Sub_Date]) Then [Com_Date] Else [Sub_Date]
Then use this variable instead of Existing submission date .
=DaysBetween([var_Sub_Date];[Cmpltd Dts] )*24*60 + ((ToNumber(Substr(FormatDate([Cmpltd Dts] ;"HH:mm:ss") ; 1 ;2))-ToNumber(Substr(FormatDate([var_Sub_Date] ;"HH:mm:ss") ; 1 ;2)))
*60) + ((ToNumber(Substr(FormatDate([Cmpltd Dts] ;"HH:mm:ss") ; 4;2))-ToNumber(Substr(FormatDate([var_Sub_Date] ;"HH:mm:ss") ; 4 ;2))) ) + ((ToNumber(Substr(FormatDate([Cmpltd Dts] ;"HH:mm:ss") ; 7; 2))-ToNumber(Substr(FormatDate([var_Sub_Date] ;"HH:mm:ss") ; 7 ;2)))
/60)