Dear IANA TZ, Since the last time change Chile has been in GMT-3 (i.e. Chile is right now in GMT-3). Chile is going to GMT-4 in May,14th midnight and returning to GMT-3 in August, 13th midnight. That means clocks will go back 1 hour at 00:00 May 15th (jumping back to May 14th); and will go forward by 1 hour on 00:00, August 14th. In tzdata2016c I can see this new rules for Chile: # Rule NAME FROM TO TYPE IN ON AT SAVE LETTER/S Rule Chile 2016 max - May Sun>=9 3:00u 0 - Rule Chile 2016 max - Aug Sun>=9 4:00u 1:00 S And I can see this Chile zone definition: # Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone America/Santiago -4:42:46 - LMT 1890 -4:42:46 - SMT 1910 Jan 10 # Santiago Mean Time -5:00 - CLT 1916 Jul 1 # Chile Time -4:42:46 - SMT 1918 Sep 10 -4:00 - CLT 1919 Jul 1 -4:42:46 - SMT 1927 Sep 1 -5:00 Chile CL%sT 1932 Sep 1 -4:00 - CLT 1942 Jun 1 -5:00 - CLT 1942 Aug 1 -4:00 - CLT 1946 Jul 15 -4:00 1:00 CLST 1946 Sep 1 # central Chile -4:00 - CLT 1947 Apr 1 -5:00 - CLT 1947 May 21 23:00 -4:00 Chile CL%sT But with these two information I'm not sure that in the midnight of May, 14th this tzdata2016c will change the TZ of Chile from GMT-3 to GMT-4. Can you please explain me how these structures in tzdata2016c work to accomplish the necessary TZ change in Chile. Thank you very much, best regards. Luis