If you’re a mac-user, then most shortcuts that would use the control key on other platforms, use the command key on Mac OS X.
So <command>+; should work in calc, at least in English UI.
Note that in some keyboard layouts the semicolon is entered using <shift>+, - that means that the keys you would have to press are <command>+<shift>+,
Also note that in that case you will not be able to enter the current date, as you have to press shift, and that then switches the shortcut to insert it as time. (<ctrl>+<shift>)+; is indistinguishable from <ctrl>+(<shift>+,)
If you want it to display the date instead, you have to format the cell to display the date instead. Or assign a different shortcut to the insert date function.