vba code top check before over-writing existing cell

[palaeontology]

I have a userform that contains a command button with the following code ...

Private Sub CommandButton11_Click()
Range("b1").Value = Now()
Range("b2").Value = Now()
NextTick = Now + TimeValue("00:00:01")
Application.OnTime NextTick, "TickTock"
End Sub

This code does two things ...
1. it timestamps the time into cell B1
2. it creates a running clock in cell B2, showing the current time.

How would I adjust that code to allow for the following two possibilities ...
1. if cell B1 already has a timestamp, provides a message that queries whether the user really does intend to over-ride the current timestamp. The message might read something like this ... "there is already a Start Time in that cell. Do you wish to over-ride it" and then allows the user to say 'yes' to overwrite it with the new current time timestamp, or 'no' to leave the timestamp as it is
or
2. if cell B1 is currently empty, proceed with the code as I currently have it (as seen above).

Kind regards,

Chris

 

[Micron]

Perhaps

Private Sub CommandButton11_Click()
Dim result As Integer

If Not Range("b1") = "" Then
   result = msgbox("There is already a Start Time in cell B1. Do you wish to over-ride it?",vbYesNo)
   If result = vbYes Then Range("B1") = Now()
End If
Range("b2").Value = Now()
NextTick = Now + TimeValue("00:00:01")
Application.OnTime NextTick, "TickTock"
End Sub

With that, it would not matter if B1 had "dog" in it so it's not dependent upon having a time value in the cell.
EDIT - I may have misunderstood that. You don't want the code to continue regardless? Just if B1 was empty?

 

[palaeontology]

Thankyou, so much, for that excellent help.

It worked perfectly,

Thankyou again !!!

Kindest regards,

Chris

 

[Micron]

Glad to help, but I thought I misunderstood the need. Thanks for the recognition.