software installed date showing incorrect

Hi

We using ITMS 7.1 SP2b

We have migrated 1000 system from win xp to windows 7

We collected these system inventory but Software installed date of these systems showing very old date.

E.g There are many system which we formated in March 2013 but after collecting these machines inventory it showing software installed date is of Sept 2012, Oct 2012, Nov 2012 Etc.

and there are many system who's software installed date is showing 2000,2002,2003