Describing it simply as "the new offset" seems like an oversimplification.

Just in BC there were multiple changes, some of which (eg Eastern Kootenays) were decided much more recently than 7 months ago. The MSTNO area didn't change, but in not changing its offset relative to the rest of the province did.

Displaying historical data, ranges that include the moment that the new definitions come into effect, temporal and/or spatial aggregations that span multiple timezone definitions... lots of tricky things can pop up.

That's not to excuse Microsoft. This isn't the first timezone change that has happened, and they should have the most robust change management process in the world for stuff like this.