Datetime - allow changes to time part but not date part

I have a parent model with ‘once-a-day’ objects and a child model with
‘many times per day’ objects. The child has a datetime field where the
date part should be obtained from the parent and the time part entered
by a user. The date part should not be modifiable by the user.

I thought I could default the datetime field on the child with the date
from the parent then use datetime_select or
select_[year|month|day|hour|minute] to permit editing of the time parts
while disabling the date parts. I’ve tried many variations without

Any suggestions? Thanks