Auto date A1 when typing in A2

I would like to automatically put the date in cell A1 when I type in A2. But I don't want to use =now() becuase it auto updates. You can get the date by pressing crtl and semicolon without updating itself but I can't figue out how to put this in a formula.

=IF(A2<>"",NOW(),"") but instead I want to use crtl + ;

Thanks.