Paul I sat down and worked my way through the UK details to see what is documented and what not. I'm normally using the results rather than the raw data, but I think I understand how this works now. I'm still using my own code in parallel with the version of TZ that Derick has built into the PHP clock tools so the main thing here is now comparing results. I've even got the original emails still archived. I've been using the 1880 'legal' date for the adoption of GMT and calling 'the other time' Railway time, but I can accept combining them. However the documents are all legal ones so using 'legal' time so midnight for a change was legal midnight not GMT until 1880 - that's politics for you :) As far as I am concerned, the Isle of Man was using LMT until 1883 and we now have a date of 30th March to go with that. I've always worked with 18 minutes offset, although Tynwald which is always quoted as the reference is 4.4814degW - 17m55.5s and that goes back 1000 years, but I think we can skip earlier solar times. So looking at the Europe data file ... # 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 Link Europe/London Europe/Jersey Link Europe/London Europe/Guernsey Link Europe/London Europe/Isle_of_Man I think I need to add ... # AT4 of 1883 - The Statutory Time et cetera Act 1883 - Zone Europe/Isle_of_Man 0:17:55 - LMT 1883 March 30 0:00s Then the London Rules apply from then so in my simple way of thinking Link To Europe/London ( date is optional as on line above? ) Fills the picture. Europe/Isle_of_Man exists in it's own right up to 1883, and then it follows Europe/London. I'm fairly sure from the IOM documents that Jersey and Guernsey followed the same date, but their archives have not got that far back yet. So do we have to duplicate the whole zone record each timezone to add this one item of data? - - Lester Caine - G8HFL ----------------------------- Contact - http://lsces.co.uk/wiki/?page=contact L.S.Caine Electronic Services - http://lsces.co.uk EnquirySolve - http://enquirysolve.com/ Model Engineers Digital Workshop - http://medw.co.uk Rainbow Digital Media - http://rainbowdigitalmedia.co.uk