macOS/OS X Numbers 3.2 inserting date / time
I believe Insert > Date &amp; Time (and its two companions) are intended for use in headers and footers. These are available after choosing File > Print. When used there, the inserted date can be clicked for a little popup formatter and button to manually reset the time to now.

The feature you want, an auto-generated NOW cell that does not change, is apparently not available. 

Option A: Put the NOW function into a cell somewhere.  It shows the current time. Copy the cell, then Edit > Paste Formula Results into the cell you want to have the unchanging time.

Option B: Use a general macro app (like the amazingly capable Keyboard Maestro) to generate your date/time text and paste it into a cell.

In both cases, choose the date and time formats you want on the final cell.

(Numbers 3.5.3 used for this answer)