From: Brian Candler <b.candler@...>
Date: 2010-08-27T19:11:52+09:00
Subject: Re: Time.local and offsets

Brian Candler wrote:
>>> Time.parse("1 jan 1847")
> => Fri Jan 01 00:00:00 -0001 1847
>>> Time.parse("1 jan 1848")
> => Sat Jan 01 00:00:00 +0000 1848
>>> Time.local("1847").utc_offset
> => -75
>>> Time.local("1848").utc_offset
> => 0
> 
> Sorry, I can't explain that one. I've heard of half-hour time zones and 
> even quarter-hour ones, but not a minute and a quarter :-)

I think I have it:
http://en.wikipedia.org/wiki/Time_zone

It wasn't until 1847 that Greenwich Mean Time was widely used across the 
UK. Before then, everyone used "solar time", which means that clocks at 
different points of the country (east to west) hit noon at different 
times. The difference would be 60 minutes for every 15 degrees of 
longitude.

So if you were standing at St Paul's Cathedral, say, noon would be 
slightly later compared to noon in Greenwich, which is a bit to the 
East.

In your case: Madrid has a longitude of 3 degrees 42 mins West. So the 
offset of Madrid solar time from GMT is:

irb(main):001:0> -(3.0 + 42.0/60.0) / 15.0 * 60.0
=> -14.8

and that's exactly what you saw. tzdata rules :-)
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