[tz] Incorrect Time Zone Data for American/Grand_Turk

[Paul Eggert]

On 08/02/2018 09:23 AM, Robert MacGrogan wrote:
> Running my own unit tests, btw, that is not the output I get. For Sun 
> Mar 11 07:00:00 2018 UT using the 2018e data I get Sun Mar 11 02:00:00 
> 2018 as the local Grand Turk time. So maybe it is a code issue. If I 
> change line 3426 in the northamerica file to this:
>
> -4:00USAST2018 Mar 11 3:00

Yes, it sounds like a code issue on your end. That change to the data is 
definitely wrong, as it would cause America/Grand_Turk to observe DST in 
2016 and 2017 (which is ahistorical) and would always abbreviate the 
time as "AST" even when it was Atlantic Daylight Time.

My guess is that your code is getting confused by the "2018 Mar 11 
3:00", and is incorrectly interpreting that timestamp as being EST 
instead of AST. As I recall, zic itself had problems in this area, long ago.