Here's how we know:
* Technology: The books feature technology like dial-up internet, flip phones, and early iPods, which were popular during that period.
* Cultural references: Greg Heffley's interests and the events in the books often align with trends and events of the early 2000s, like the popularity of "Harry Potter" and "Lord of the Rings."
* Author's statements: Jeff Kinney, the author, has confirmed in interviews that the story is set in a "time when the internet was still relatively new."
Therefore, the "Diary of a Wimpy Kid" series likely occurs sometime between 2003 and 2007, aligning with the time period when the books were originally published.