If the constituent after 'is' answered the question 'when', would that mean 'is' can't be a linking verb in that case? No! Linking verbs can be followed by adverbials (SVA; V = copula), which is not the matter at hand, though.
What is the time?
the time is that, 10 pm. Nominal!
When is the time?
The time is 10pm.
The time is 10pm.,
'is' is a linking verb, a copulative verb, a copula.