Hi,
I'm not sure if this is a bug or not, but it appears that when the default value is removed from the "Newer than" textbox on the search page, then the search is performed on all topics that have ever been made and it is not limited to the maximum that the board has (in my case 6 months). I'm not sure if this is a bug with Infopop or a bug with the search modification that I got from threadsdev.com.