How to write a code so that a macro called "Mega" runs only once if cell value A1 > 0

[omega0010]

Apologies for A1>0 above. It should be if cell value [A1] > 00:00:15 ( ie 15 seconds). How to write a code so that a macro called "Mega" runs only once if cell value A1 > 00:00:15 ( time ), EVEN THOUGH the cell value (A1)remains increasing after that. The macro called "Mega" will also reset the value of cell A1 to zero which will start increasing with time but the macro ( Mega ) should not run for the second time. Please see the attached file. Once the macro "Mega" runs once and resets the cell A1 will start increasing with time but next time when it crosses 15 seconds, it should not trigger by itself( ie by the code which we write( ideally a simple code like

Private Sub Worksheet_Calculate()

If [A1] > Time(0,0,15) Then
Mega
End If
End Sub

......will run it , but then there is nothing to stop it from running again. What can we do to change this code so that Mega may run only once ?

[TMS]

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Regards, TMS

[omega0010]

Thanks for your help. Unfortunately the code does not work alone or in combination ( I tried to put it with the code below because I need to run them together). I have put it in the main sheet code.
Strangely usually I am using this code given below successfully with many steps and macros elswhere( just one of them being below)-

Private Sub Worksheet_Calculate()

If [A1] > Time(0,0,15) Then
Mega
End If
End Sub

How can we ad your code to the code above ? ( or can it run separately from a module ?)
Actually while this simple code gives accurate and smooth run with many variables and steps (macros) elsewhere , in this particular sheet it gives me a prompt saying "Run time error" Type mismatch ..why ? Then how does this code run with many macros elsewhere ? Kindly throw some light on this subject. Thanx
Omega0010

[TMS]

In Sheet1 module:

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In standard/normal module:

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See the updated example

Regards, TMS

Edit: trying to make thread appear in lists (second attempt)

[TMS]

No new content: Edit: trying to make thread appear in lists (third attempt)

[omega0010]

Thanks a lot TMS.
I really appreciate your help.What appears to me is that nothing is happening despite the sophisticated programming done by you( which is far from my understanding of course!). I mean , I feel that from the opening of the file the program remembers that the macro was run at some time prior to opening time because when i try to trigger it by changing any different cell value the the time appears to be changing but nothing happens. Like how do we test that the program i.e. Mega has run once ?
Thanks always to keep teaching people like me and helping the helpless!

[omega0010]

Yes I have checked it and it actually works as immediately as the book opens where as actually my idea was to open the book at around 9 am computer time and run mega at 9:15 for once could you please give a direction as to what change to do ?