The moment when the date changes (8)
I believe the answer is:
midnight
'the moment when the date changes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for midnight that I've seen before include "Possible time for feast" , "12 in the evening" , "12 hours from noon" , "when it's usually dark" , "type of oil burnt by late work" .)