The override function does not work.

It always says

"Override unavailable. No override period has been set in the options."

I have both checked and unchecked the "allow temporary override for these sites" options and looked into the Advanced Settings but nowhere is there a place to set that period option.
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RESPONSE TO DEV RESPONSE:

AH! Thank you! I'd suggest explaining that in the error message -- that small copy change would have saved me a LOT of pain.