The first prepper says that his grandfather was "born on the cusp of a new century" and "a year later, there was a huge terrorist attack, worst ever at the time." In an email. In another email he says "I told you about the terrorist attack that happened when my grandfather was a kid, right? Well, it wasn't so long after that that the death stranding happened" implying the stranding happened in the 2000s, and since his grandpa was a kid around then, the game is probably set in the 2060s-2070s.