Stop entry overwrite and clear cell data off of entry in another cell

I am trying to get a VBA code to automatically place the current time in cell F1 by any entry in cell A1, but can’t be over written once entered if the user enters something in the same A1 cell again. And the same to place the current time in cell G1 by any entry in cell L1, but can’t be over written once entered if the user enters something in the same L1 cell again. Then if a value is changed in L1 to one of two values "TA" or "FW", cell H1 will be blanked of any entry.

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I can get the time stamp to work for both column "F" and "G".

I can't seem to get the check of column "H" and clearing the cell if the entry in column "L" is changed to the value of "TA" or "BT" to work.

I am a newbie so I would appreciate if you put the comments in as to what each line is doing so I can understand what is going on.