I have a spressheet that automatically populates 2 cells with the current
time and date using the TODAY() and NOW () functions. Unfortunately, when I
go back into the sheet at a later date or time it repopulates the cells with
the new time and date.

e.g. First Visit date set to 5-Jan 05 and time set to 09:20 AM.
Second visit date set to 5-Jan 05 and time set to 09:25 AM.
Third visit date set to 6-Jan 05 and time set to 10:00 AM.

Is there a way that I can only get the cells populated once?

Any help would be appreciated.