Setting date of column if another not blank

I need to create a table where column A is automatically populated with a date and time when column B is input for the first time. The date in column A should not change if the contents of column B change only when it is input the first time.

I started with column A created like so:

=IF(B7<>"",TEXT(NOW(),"hh:mm:ss"),"")

Which creates the date but it stays dynamic and updates to the current date whenever any cell is updated.

I cannot use VBA as this needs to run on the MAC version of Excel too.


Thanks in anticipation....