We came across this problem today: Replies in Apple Mail app always show the same (wrong) sender date like "On 26.09.2007, at 14:08, Tom Jones wrote:" and its always 26.09.2007 in every reply to any sender, no matter on what date they had really sent their e-mail.

The solution to this is in System Preferences > International > Formats
Change the Region from "Custom" to your local Region e.g. "Switzerland"