They do. The problem is that the rules of DST change over time (sometimes at the very last minute https://github.com/eggert/tz/commit/868f3528a9fd60491439ce457f

They do. The problem is that the rules of DST change over time (sometimes at the very last minute https://github.com/eggert/tz/commit/868f3528a9fd60491439ce457f6d68dc12ef425b https://github.com/eggert/tz/commit/868f3528a9fd60491439ce45...) and can lead to all sorts of date math bugs when comparing timestamps across timezones.