Instead of using vba just enter the formula for the first row on C1 :
=ROUND(a1,0)-ROUND(b1,0)
Then just the formula to the end of exisiting rows.
If you insist using vba code use the simple code below:
Dim LastRow As Long
LastRow = ActiveSheet.Range("A" & Rows.Count).End(xlUp).Row
Range("c1").Select
ActiveCell.FormulaR1C1 = "=ROUND(RC[-1],0)-ROUND(RC[-2],0)"
Range("c1").AutoFill Destination:=Range("C1:C" & LastRow)
End Sub
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solved How to find the days difference between two dates [closed]