isdst.md

Investigations show that ALL timezones that observe DST are in DST

• Jan 1st (southern hemisphere) or
• July 1st (northern hemisphere)

No binary search is required to simply know if a given moment is observing dst.

This allows us to imagine the following algorithm

var getTimezoneOffset = require('get-timezone-offset');

  module.exports = function( tz, date ) {
    var d = new Date(date);
    var offset = getTimezoneOffset( tz, d );
    var jan = getTimezoneOffset( tz, new Date(d.setMonth(0, 1) );
    var jul = getTimezoneOffset( tz, new Date(d.setMonth(5, 1) );
    var dst = Math.abs( jan - jul ) === 60 && Math.min( jan, jul );
    return {
      hasDst: !!dst,
      isDst: offset === dst,
      northern_hemisphere: dst === jul,
      southern_hemisphere: dst === jan
    };
  }

However this fails when the offset for a timezone changes permanently during the year, which happens a lot

A test which checks a single time each day for every timezone for a year takes about a minute to run.

The algorithm could be improved by testing jan/jul for a year each side and drawing conclusions from that

Also

• Singapore had a DST of 20 minutes in 1933
• Namibia observe Winter Time which effectivly puts the rest of the year in summary time
• UK in 1940-45 shifted timezones and observed DST, which is known as double daylight savings
• a bunch of other weird time oddities

In anycase you can get close with a 12 line program or closer with a larger program, but it won't be 100% accurate.

Hemisphere calculations will both be false if the timezone is not shown to be in DST