What is the significance of the "date designation expires" on a temporary airman certificate?
"date designation expires"


Assuming your temporary certificate is issued in the United States.

Is the date that the Designated Examiner's designation expires (assuming it was a "Designated Examiner" who issued the temporary certificate).

A Designated examiner's authority has to be periodically renewed. 

This is not the date your temporary certificate expires, which is 120 days from issuance.