Item modification dates, is NS doing something to reset them?

Are the "Item modification dates" working?  When I go to the list of our items (5000+) they are half set to a modification date of 12/14/2006 and the other half are set to 10/13/2006.

We have made some changes to the CATEGORIES lately...will that effect the modification dates of the items displaying within those categories?  We export froogle feeds...can an export effect the modification dates of the items being exported?

I know the dates I am seeing are not accurate because we modify items practically every day and they wouldn't be so uniform (half one date and half another date) and they should almost all have different modification dates (practically).  I am wondering if NS had to do a export/import to fix a problem (causing the large groups with the same dates).  Also, the times on these modification dates is times when we don't work (like 4am).