Disabling a Time Function

I searched the forum and couldn't find an answer to this.

I'm creating a system that will allow staff to scan their work-card to show when they arrived. What I need is a cell that dynamically gets the current system time and enters it into a cell. Once this time value is entered I would like the cell to become just a value i.e. no function.

Here's what I have.

I have a cell with the formula -

=IF(ISBLANK(D9),"FAIL",showTime())

The showTime function is in the Module

Function showTime() As Variant
showTime=Now
End Function

This works correctly on the first usage but then the value gets updated every time the sheet opens and sometimes when I'm editing other Functions.

So is there a way of making a function run once, or disabling a function, preferably I would like to change the cell contents to a value.

Any help would be appreciated.

Keill