Is the deadline for filing if you had an extension Oct.15 at midnight?

Yes, midnight your time zone.

https://ttlc.intuit.com/community/tax-planning-and-deadlines/help/tax-year-calendar-important-dates-and-deadlines/00/26148 

Yes; just make sure if you are mailing that return is postmarked on or before that date.

If you will be e-filing then waiting until just before midnight may be a frustrating experience for you if anything goes wrong.  If it rejects you will have a few more days to fix the problem and try again, but after that the return will have to be printed, signed and mailed.