Change your formula as follows and drag down

K6=IF(W6="",IFERROR(V6-A$1,""),W6)

Change the number format of the cells from K6 to mm:ss

And replace your routine with this

Code:
Sub RunEveryMinute()
    With Worksheets(1)
        Range("A1").Value = Format(Now, "hh:mm:ss")
    End With
    If Range("P6").Value = "Completed" Then
        Range("W6").Value = Range("K6").Value
    End If
    If Range("K6").Value < TimeValue("00:30:00") And Range("P6").Value <> "Completed" Then
        MsgBox "Alert! Alert! Abort Mission!. This file will explode in " & Range("K6").Text & "  minutes. Turn computer off if you want to live"
    End If
    Call OnTimeMacro
End Sub