How to calculate Turn Around Time (TAT) considering weekend and working Hours..!

Ateeb 

Assuming that's Excel and assuming Start/End could be outside the working hours, for such sample

formula in C2 could be

=IF(INT(B2)>INT(A2),
  NETWORKDAYS.INTL(A2+1,B2-1,"0000011",$J$2:$J$3)*8/24 +
  (WORKDAY.INTL(A2-1,1,"0000011",$J$2:$J$3)=INT(A2))*
  (
    $F$2-MEDIAN($E$2,$F$2,MOD(A2,1)) +
    $H$2-MEDIAN($G$2,$H$2,MOD(A2,1))
  ) +
  (WORKDAY.INTL(B2-1,1,"0000011",$J$2:$J$3)=INT(B2))*
  (
    MEDIAN($E$2,$F$2,MOD(B2,1))-$E$2 +
    MEDIAN($G$2,$H$2,MOD(B2,1))-$G$2
  ),
  MIN($F$2,MOD(B2,1))-MEDIAN($E$2,$F$2,MOD(A2,1))+
  MEDIAN($G$2,$H$2,MOD(B2,1))-MAX($G$2,MOD(A2,1))
)

Result formatted as [hh]:mm.

Firtst we check if Start/End dates are different days, if so calculate number of full workdays in between plus time for each of Start/End if only they are workdays. Finally calculate the difference if Start/End is the same date.