[tz] SGT abbreviation is missing from the IANA tz database

[Paul Eggert]

On 02/19/2018 06:55 PM, Lu, Can via tz wrote:
> SGT is very commonly used in Singapore and it’s also a standard 
> abbreviation for Singapore time. Please refer to: 
> https://en.wikipedia.org/wiki/List_of_time_zone_abbreviations and 
> https://en.wikipedia.org/wiki/List_of_time_zone_abbreviations

Those two sources are not reliable for real-world use, as they're 
derived from old versions of the tz database itself. If SGT is commonly 
used independtly of tzdata (in the main newspapers of Singapore, 
government sources and the like), that would be good evidence for its 
inclusion in tzdata. My impression, though, is that it's not that common 
in practice; though I found some occurrences in Singaporean sources, it 
wasn't all that common, perhaps in part because SGT also stands for 
"Sergeant", a use far more common than the time-zone use.