I received an email today inviting for an interview. The interview was scheduled for 8 days ago, I am using Gmail so there is no way this was a fault on my part. How should I answer in this email?
Check the email headers to see when it was sent, and where it spent all that time. It's unlikely that it was in-transit for over 8 days, as it would have automatically been returned to the sender as undeliverable after about 3 days.
You'll probably find that it was sent today (or maybe late last week) - so contact the company and ask if the interview date is incorrect, given the sending date of the email.