Fix issue #182: wrong offset within 1hr of DST start

[bsvetlik]

Solved the Chicken and Egg Problem described in summerTime.js: "we can't get the date, without knowing the timezone, and vice-versa".
In order to eliminate the time zone offset effect the given UTC time is compared to the DST change boundaries converted to UTC.

[spencermountain]

holy crap!
i would hug you if I could.
gonna merge this into develop and throw a bunch of tests at it - I guess the trick is using Date.UTC()?
That is an extremely clever solution.
WIll play-around with this this afternoon, and check back.
Thank you, so much.

[bsvetlik]

Hi,
I'm glad I was able to help!
A few days ago I solved the similar problem: change JS Date object to show time in another time zone. I evaluated a number of date-time libraries that support DST changes (moment, luxon, date-fns-tz) but they were amazingly slow (0.5sec for 10,000 conversions). I had to find other solution: given the specific time zone offsets with DST change boundaries received from the server as UTC milliseconds shift the time by corresponding offset. I developed an algorithm that was much faster (15 milliseconds for 10,000 conversions). The spacetime library issue #182 was similar to my problem. The only difference was the DST change boundaries in the specific time zone. After converting them to UTC the Chicken and Egg problem (I liked your description - it was in tune with my thoughts when I worked on conversion algorithm) was solved!
Regards. Boris.

[spencermountain]

this works amazingly.

published as 6.8.0
🏅