[tz] Ethiopia local time

[Paul Eggert]

On 10/11/18 7:17 AM, Brian Inglis wrote:
> May want to emphasize that this approach only works/should be used if both zones
> observe year round standard time with no DST as in equatorial zones.

It can work to some extent even if DST is being observed. For the DST 
rules for Sudan 1972-1985, for example, 
TZ='<-04>4<-03>,M4.5.0/-6,J288/-6' should work on tz-compatible 
platforms. Admittedly this sort of thing is not for the fainthearted, 
nor will this particular example work with common cellphone UIs or on 
strict POSIX platforms. It's probably not worth going into all this in 
theory.html, though, so I installed the first attached proposed patch.

While looking into this I discovered that zic.c does not allow negative 
times in Rule lines, contrary to the documentation since 2018a. Negative 
times would be needed to support Ethiopian-style timekeeping in zones 
where the European-time transitions are betwee 00:00 and 06:00, as in 
the case in Sudan 1972-1985, where the zic input might have looked like 
this:

     Rule Sudan-Ethiopian-style 1972 max - Oct 15      -6:00 0 -
     Rule Sudan-Ethiopian-style 1972 max - Apr lastSun -6:00 1:00 -
     Zone Africa/Khartoum-Ethiopian-style 2:10:08 - LMT 1931
      -4:00 Sudan-Ethiopian-style -04/-03

I fixed this by installing the second attached proposed patch.

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