There is one insidious domain mail server (a US government site) that wreaks havoc with the dates. Default Repair Folder only makes it worse updating to current date and time. This add-on fixes that. After the add-on using Repair Folder is still impaired by what is shown in the "Date" column, but the "Received" column is perfect!
Hopefully Mozilla can update a fix to this issue.
This review is for a previous version of the add-on (1.0).