Here are some clues:
* Technology: The books mention cell phones, the internet, and video games, but these technologies are not as advanced as they are today. This suggests a time before widespread use of smartphones and social media.
* Social context: There are references to things like the Y2K scare and the popularity of reality TV shows, which were prominent in the early 2000s.
* Author's intent: Trenton Lee Stewart has said in interviews that he intended for the books to feel timeless, but they are definitely set in a time that is not too far in the past.
Based on these clues, it's safe to say that the Mysterious Benedict Society books likely take place sometime between 2000 and 2005.