[tz] Irish time...

[Joseph S. Myers]

On Sun, 16 Oct 2011, Kevin Lyda wrote:

> The first entry in the europe zone file for Ireland is as follows:
> 
> Rule    GB-Eire 1916    only    -       May     21      2:00s   1:00    BST
> However I don't think it's correct.  I'm not sure how far back we want
> to go, but apparently from 1880 up until October, 1916, Dublin was 25
> minutes off from London.  When the clocks were set back in October
> 1916, they were set back 35 minutes - which would seem to cast doubt
> on the second line for GB-Eire:
> 
> Rule    GB-Eire 1916    only    -       Oct      1      2:00s   0       GMT

The effect as shown by zdump appears to be correct:

Europe/Dublin  Sun May 21 02:25:20 1916 UTC = Sun May 21 01:59:59 1916 DMT isdst=0 gmtoff=-1521
Europe/Dublin  Sun May 21 02:25:21 1916 UTC = Sun May 21 03:00:00 1916 IST isdst=1 gmtoff=2079
Europe/Dublin  Sun Oct  1 02:25:20 1916 UTC = Sun Oct  1 02:59:59 1916 IST isdst=1 gmtoff=2079
Europe/Dublin  Sun Oct  1 02:25:21 1916 UTC = Sun Oct  1 02:25:21 1916 GMT isdst=0 gmtoff=0

That is, the clocks went forward one hour at 2am DMT on 21 May, and then 
at 2am DMT (3am summer time) on 1 October they went to GMT.  See attached 
scans of the relevant laws.

The relevant UK National Archives file has a public information poster 
used in Ireland to inform people of the 35-minute clock adjustment.  What 
times people were actually using in Ireland, I don't know.

I have not attempted a thorough survey of Irish laws relating to time; my 
2005-01-26 comment in the europe file lists those I found (though those 
URLs seem to have become broken since then) but I haven't attempted to 
match them up to individual transitions and add comments for each 
transition giving the Irish legal basis for it alongside those giving the 
British basis.

-- 
Joseph S. Myers
jsm at polyomino.org.uk
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