Not all the time that's in the future (4,3)

I believe the answer is:

some day

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'in the future' is the definition.
Both the definition and answer are adverbs. Perhaps they are linked in a way I don't understand?

'not all the time' is the wordplay.
'not all' becomes 'some' ('some' is the opposite to 'all').
'the time' becomes 'day' (day is a kind of time).
'some'+'day'='SOME DAY'

'that's' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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