Byzantine calendar
The calendar used by the Eastern Orthodox Church from c. 691 to 1728
Did Ivan the Terrible use the Byzantine calendar?
Ivan the Terrible was the Tsar of Russia from 1530 to 1585.  The Byzantine calendar was the official calendar of the Russian government from 988 to 1700. The Tsar was the leader of the Russian government. 
What was Ivan the Terrible's role from 1530 to 1585? What country was Ivan the Terrible #1 of? Was the Byzantine calendar the official calendar of #2 from 1530 to 1585?