Today formula function won't update automatically

I have a numbers document created on my mac. Every time I open it, I want the cell with the "today" function to reflect today's date. But it doesn't, it reflects the date when I last opened the document (and corrected it by deleting the today function, closing the cell, reopening the cell, and re- adding the today function, quite inconvenient. First world problems....)

How can I have the cell automatically look up today's date every time i open the document? thanks!