[tz] Re: Past changes in Mexico

Thanks for that detailed study of Mexican decrees. I transcribed your results into TZDB format and installed the attached proposed patch into the developmental repository on GitHub. One thing came up when I was reading your comments. You wrote "Quintana Roo changed to UTC-6 on 4 January 1983 at 0:00, and again to UTC-5 on 26 October 1997 at 2:00." In the attached patch, I'm taking this to mean that because America/Cancun observed Mexican-style DST in 1996 and 1997, its 1997-10-26 02:00 "transition" was from CDT (-05, or -06 with DST) to EST (-05), i.e., civil-time clocks didn't actually jump that day. If this understanding is incorrect please let us know. Thanks again.