Calendar Change

In 1582 Catholic countries and empires started using the Gregorian Calendar (created by Pope Gregory XIII) to fix an error in the Julian Calendar. Most Protestant and other Christian and Muslim countries and empires saw it as a Catholic ploy and so took several hundred years to switch.

The United Kingdom (along with the American and Canadian colonies) didn't switch until 1750. Communist Russia didn't switch to it until 1918 (that's why the Great October Socialist Revolution is actually on November 7th.)

Some countries use other calendars alongside the Gregorian calendar, including: India (Indian National Calendar), Bangladesh (Bengali Calendar), Nepal (Vikram Samvat), Pakistan (Islamic Calendar), Israel (Hebrew Calendar) and Myanmar (Burmese Calendar), and other countries use a modified version of the Gregorian calendar, including Thailand (Thai Solar Calendar), Japan (Japanese Calendar), North Korea (North Korean Calendar) and Taiwan (Minguo Calendar.)

Today there are three countries that do not use the Gregorian Calendar: Ethiopia, Iran and Afghanistan.

Just one more thing to remember when you are making trees and using dates as though the constantly changing names of places/countries/etc. wasn't enough.