The Holocene calendar is a way to measure and keep track of time that starts in the year 10,000 BCE (Before Common Era). This calendar is a bit different than the calendar we use today, because it based off of when the Earth's climate changed from the last Ice Age. Instead of having months and years, the Holocene calendar is divided into 5 "ages". Each of these ages is 12,000 years long, and the way they are divided changes depending on what parts of the world you're looking at. For example, the first age is split up differently in Europe than it is in North America. Each age is then split up into smaller eras and periods, which are a little like months and years, but much longer.
Using the Holocene calendar helps people to better understand our changing climate and how it has affected different parts of the world. It can also be used to compare events from different time periods so we can learn more about our past.