Etiquette Question - Professor asked if I could meet after X time on Y day. I emailed to confirm a specific time but he has not responded I have been looking to get started in undergraduate research (CS) and finally got a bite on a cold email. This professor said he read my resume and "liked my profile" and asked if I could meet him after X on day Y. I responded asking if a specific time in that time frame worked for him. However, it's been a day and a half and he has yet to respond. (the day is tomorrow)

If he does not respond today, would it be rude to knock on his door during the time frame he specified? Or does the lack of response suggest he has rethought the offer?
No, it would not be rude to knock on his door. He's already said the time is OK! However, I would not have worded the email asking him if [time] is OK - I'd simply say something like "alright, I'll be there at [time]", which saves him from having to respond.

If he indeed rethought the offer, it'd be quite unethical of him not to tell you before you visit him. But you knocking on his door won't hurt anyway: the worst that can happen is that he tells you then that he's rethought the offer.