Adding calendar qualifications like that to the times makes it even more complicated. To use another commenter's example about "valid hours of parking" printed on narrow street signs, there's no space to append long conditional text (like your suggested verbiage) to the times.

Also, the DST change date isn't stable. Over the years, the DST date boundaries have changed: from the last week in April -- then to first week in April -- then to March. The DST rules may further be refined to be always or never.

Did you look at my link to my previous message? The business hours on the window was only one example of complexity.