This macro will give you the elapsed time in days.
Option Explicit
Sub ElapsedTime()
Dim x As Long
Dim LastRow As Long
Dim StartTime As Double
Dim EndTime As Double
LastRow = Range("A65536").End(xlUp).Row
For x = 2 To LastRow
Select Case Range("C" & x).Text
Case Is = "ENTERED"
StartTime = Range("D" & x).Value
Case Is = "BILLED"
EndTime = Range("D" & x).Value
Range("E" & x).Value = EndTime - StartTime
Case Else
Range("E" & x).Value = Range("D" & x).Value - Range("D" & x - 1).Value
End Select
Next x
End Sub