If history happens a couple of million/billion years ago, then why is the year only 2021?

well, AD and BC. The terms Anno Domini and Before Christ are used to label or number years in the Gregorian calendar. we are in AD.

lee and grant, both of whom held the highest rank in their respective armies, had known each other slightly during the mexican-american war (1846-48) and began their dialogue by exchanging awkward personal inquiries. characteristically, grant had arrived in his mud-splattered field uniform while lee had turned out in full dress attire, complete with sash and sword.

explanation:

the communist manifesto is an 1848 political pamphlet by the german near the end of the manifesto, the authors call for a "forcible overthrow of all existing social conditions",  ..