In another theory, date was the very beginning of time (3,4,3)
I believe the answer is:
the year dot
'in another theory date' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'the very beginning of time' is the wordplay.
'very beginning of time' becomes 'year dot' (I've seen this before).
'the'+'yeardot'='THE YEAR DOT'
'was' acts as a link.
Can you help me to learn more?
(Other definitions for the year dot that I've seen before include "date, one imagines" , "Time's beginning" , "As long ago as can be remembered" , "right at the start" , "A very long time ago" .)