Dear sir/madam On this website i find the following data: https://www.timeanddate.com/time/zone/uk/london 2015Sun, 29 Mar, 01:00GMT <https://www.timeanddate.com/time/zones/gmt> → BST <https://www.timeanddate.com/time/zones/bst>+1 hour (DST start)UTC+1h Sun, 25 Oct, 02:00BST <https://www.timeanddate.com/time/zones/bst> → GMT <https://www.timeanddate.com/time/zones/gmt>-1 hour (DST end) <https://www.timeanddate.com/news/time/europe-dst-end-2015.html>UTC 2016 Sun, 27 Mar, 01:00GMT <https://www.timeanddate.com/time/zones/gmt> → BST <https://www.timeanddate.com/time/zones/bst>+1 hour (DST start) <https://www.timeanddate.com/news/time/europe-starts-dst-2016.html>UTC+1h Sun, 30 Oct, 02:00BST <https://www.timeanddate.com/time/zones/bst> → GMT <https://www.timeanddate.com/time/zones/gmt>-1 hour (DST end) <https://www.timeanddate.com/news/time/eu-debate-dst.html>UTC 2017 Sun, 26 Mar, 01:00GMT <https://www.timeanddate.com/time/zones/gmt> → BST <https://www.timeanddate.com/time/zones/bst>+1 hour (DST start)UTC+1h Sun, 29 Oct, 02:00BST <https://www.timeanddate.com/time/zones/bst> → GMT <https://www.timeanddate.com/time/zones/gmt>-1 hour (DST end)UTC 2018 Sun, 25 Mar, 01:00GMT <https://www.timeanddate.com/time/zones/gmt> → BST <https://www.timeanddate.com/time/zones/bst>+1 hour (DST start)UTC+1h Sun, 28 Oct, 02:00BST <https://www.timeanddate.com/time/zones/bst> → GMT <https://www.timeanddate.com/time/zones/gmt>-1 hour (DST end)UTC 2019 Sun, 31 Mar, 01:00GMT <https://www.timeanddate.com/time/zones/gmt> → BST <https://www.timeanddate.com/time/zones/bst>+1 hour (DST start)UTC+1h Sun, 27 Oct, 02:00BST <https://www.timeanddate.com/time/zones/bst> → GMT <https://www.timeanddate.com/time/zones/gmt>-1 hour (DST end)UTC As you can see, the Europe/London time change rule is different every year . But i can not find this information on yor data distribution. https://www.iana.org/time-zones For europe, london you give: # Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Europe/London -0:01:15 - LMT 1847 Dec 1 0:00s 0:00 GB-Eire %s 1968 Oct 27 1:00 - BST 1971 Oct 31 2:00u 0:00 GB-Eire %s 1996 0:00 EU GMT/BST Maybe i do not understand how to use your data? I put the question on stackoverflow: http://stackoverflow.com/questions/40360032/iana-internet-assigned-numbers-a... regards, -- *Gintare Statkute*