While this construction is common in colloquial speech, the subordinate clause introduced by "when" is an adverb, so it can't modify the noun "time."
I think.
Why is the sentence below incorrect?
The mailman came at a time when it was impossible for me to get the mail.
2 answers
Steve is right.
If you substitute 'at which' for 'when,' the sentence will be fine.
If you substitute 'at which' for 'when,' the sentence will be fine.