That site contains the MD_DS3231 library but that's not the one he shows in the code in the video. And that's also not the one you're using. But anyway that one does not have a getTime method either. It has readTime but it's used completely differently to the code you have.
If you understand what the code is trying to do you should be able to change it to use MD_DS3231.h but it's a fairly major change.
After a lot of Googling it looks to me as though the original Eric Ayars DS3231.h from 2011 had a getTime() but it has been taken out of the many later "improved" versions. You could try loading the original library from Hardware Hacks: DS3231 Real-Time Clock in place of the one you currently have and see if that works. Oh and remove the include of MD_DS3231.h unless you're going to use it.
Steve