Robin or Doris to be chosen in time (3,2,5,4)

I believe the answer is:
one of these days
'in time' is the definition.
(synonyms)
'robin or doris to be chosen' is the wordplay.
I cannot really understand how this works, but
'doris' could be 'day' (actress Doris Day is an example) and 'day' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for one of these days that I've seen before include "At some time in the near future" , "Some time soon" , "Sooner or later" , "provide a selection for a date in the diary?" .)
