depending on what you want exactly, there are two possible methods.
To enter the current date and not have it change the next time you open the file, use control-; (semicolon). Also, to enter the time instead, use command-;
to enter the current date and have it autoupdate each time you open the file (good for templates or invoicing, etc), use the function =today(). If you enter that (and that alone) in a cell, that cell will always display the current date, updating each time you open the file.
Jw