Here is a test script that looks for "Insert Date" in cell Sheet 1::Table 1::A1 and will overwrite it with the date. If you pre-format the cell as Date&Time with the date format you want, it will display it that way. It is checking every 2 seconds.
tell application "Numbers"
repeat
try
tell front document
if value of cell "A1" of table "Table 1" of sheet "Sheet 1" is "insert date" then
set value of cell "A1" of table "Table 1" of sheet "Sheet 1" to date string of (current date)
end if
end tell
end try
delay 2
end repeat
end tell
Open the Script Editor app. Copy/paste the script to a new document. Hit the Play button. In Numbers, put "insert date" into that cell and it should change within 2 seconds. Hit the Stop button in Script Editor when you're done trying it out.
A script like this can be turned into an application that can be one of the log-in items to load and run when you log in.