It was a scam call - 100% definite. I hope you didn't give out any personal details as to bank card, etc. He probaby guided you to a part of the Registry which would show you a list of expired Certificate which were only indications of the security of certain websites. When you saw the expiry dates, it boosted his argument that your software was out of date.
Another scam is where they ask you to look in a certain key in the Registry where there's a string of alpha numeric characters which appear in every installation of Windows. He will have given you tyhat string of characters and when you said they were in your Registry, he's got you worried.
Hang up as soon as these con-artists start speaking or if you want to have some fun, keep him on the phone for half an hour then say "Are you sure my copy of Linux Mint has this string/certificate/whatever?". You'll be helping to protect someone else.