Auto insert date based on condition of another cell

i wanted to "auto insert the current date in cell A1 IF A2 has any data in it. i did that successfully as follows:

 =IF(A2<>"",NOW(),"")

BUT,...... it changes every day or maybe its changing every time i load the workbook. i need it to insert at the time of data entery into A2 and then stay there from then on out. any ideas?? I only want NOW() to execute once.