...the OLD DST dates are HARD CODED in the CM15A's firmware. If it is not connected to the PC, it will use the old dates.
I'm using the BMB Europe version of AHP V3.251 (I believe) with my U.S. CM15. I didn't get any time shift over the weekend of April 3,4 and my CM15 was operating on its own. I have a chime set to go off 3 times a day to check the clock accuracy, and I would have noticed it. Other people have said that the BMB version doesn't have DST, and my experience so far seems to confirm that.
If I remember correctly, the old AH software for the CM11 allowed the user to check whether or not to use DST.
As Dan mentions above, DST is hard coded in the CM15. It seems that for my CM15 (using BMB software), not to respond to DST dates, there must be a flag in AHP that enables the DST shift dates. If that flag is turned on in the U.S. version and not turned on in the Eu version, then the version of AHP you use must determine whether you have DST active or not, regardless of wheter the DST dates are contained in the .ahx file or ROM in the CM15.
So far, this is just a guess, since I've had enough problems with AHP, the CM15 and my house wiring to turn my hair more grey than it already is. I get another chance to test the theory in a few months.