"In the evening"
King says in his grammar (§454, s.v. efo):
Efo (in some areas hefo) is the general word for with in the N. In the S it is always replaced by gyda, a word which, in turn, is virtually unknown in the N (except in the set phrase gyda’r nos at night).
Note the last bit in parentheses :)
No, see earlier comments. gyda'r nos is an idiomatic expression meaning 'evening' and 'in the evening'. Another example of where a literal translation does not work.
The hints are not context-sensitive, so just because they say "noswaith = evening" does not mean that that is true in all situations.