This requires an event macro. Assuming that the cell is A1, put this in
your worksheet code module (right-click the worksheet tab and choose
View Code):

Private Sub Worksheet_Change(ByVal Target As Excel.Range)
With Target
If .Address(False, False) = "A1" Then
Application.EnableEvents = False
.Value = 2 * .Value
Application.EnableEvents = True
End If
End With
End Sub


In article ,
"johnp" wrote:

The cell would have a time format of 00:00:00 or 00:00.
If one entered 01:00:00 into the cell, the cell would show 02:00:00.
If one entered 00:00:32 into the cell, the cell would show 00:01:04.