Here's how we know:
* Technology: The books feature technology like flip phones, dial-up internet, and early video game consoles, common in that era.
* Social Trends: The books depict fashion, music, and social trends consistent with the early 2000s.
* Historical Events: The books mention events like the 2004 presidential election, which further places the timeframe.
While the exact year isn't explicitly stated in the books, it's clear that Greg Heffley's experiences reflect a middle school life in the early 2000s.