When does a new decade actually start?

According to a new survey, 64% of people say the new decade starts on January 1st, 2020 and ends on December 31st, 2029.  But 17% say it starts on January 1st, 2021 and ends on December 31st, 2030.

The best argument for the 2021 start:  There was no year zero, so the first decade was 1 A.D. to 10 A.D.  The best argument for the 2020 start: isn’t it obvious?