Hi, i need a small code with now().

When i write some text in A1, the time to appear in B1. But i need the time
to appear just once. If i close the workbook and open it again, the time to
remin intact. I want the code to work just once. And olso, if i add something
in A1 the code to remain intact.
If the first time i write something in A1 the time will be 12:00, then this
time to remain displayed all the time.
can this be done?
Thanks in advance.