dst/index.js

determine if a date is in daylight savings time

;(function() {

// so, the only way we (reliably) get access to DST in javascript
// is via `Date#getTimezoneOffset`.
//
// this value will switch for a given date based on the presence or absence
// of DST at that date.

function find_dst_threshold (near, far) {
  var near_date = new Date(near)
    , far_date = new Date(far)
    , near_offs = near_date.getTimezoneOffset()
    , far_offs = far_date.getTimezoneOffset()

  if(near_offs === far_offs) return 0

  if(Math.abs(near_date - far_date) < 1000) return near_date

  return find_dst_threshold(near, near+(far-near)/2) || find_dst_threshold(near+(far-near)/2, far)
}


function find_dst_thresholds() {
  var d = new Date()
    , d = new Date(d.getFullYear(), 0, 1)
    , f = new Date(d.getFullYear(), 11, 31)
    , x
    , first
    , second

  x = (f - d) / -2
  first = find_dst_threshold(+d, d - x)
  second = find_dst_threshold(d - x, +f)

  return {
    spring_forward  : first ? (first.getTimezoneOffset() < second.getTimezoneOffset() ? second : first) - new Date(d.getFullYear(), 0, 1, 0, 0) : 0
  , fall_back       : first ? (first.getTimezoneOffset() < second.getTimezoneOffset() ? first : second) - new Date(d.getFullYear(), 0, 1, 0, 0) : 0
  }
}

var THRESHOLDS = find_dst_thresholds()

function is_dst(datetime, thresholds) {

  thresholds = thresholds || THRESHOLDS

  if(thresholds.spring_forward === thresholds.fall_back)
    return false

  var offset = datetime - new Date(datetime.getFullYear(), 0, 1, 0, 0)
    , dst_is_reversed = thresholds.spring_forward > thresholds.fall_back
    , max = Math.max(thresholds.fall_back, thresholds.spring_forward)
    , min = Math.min(thresholds.fall_back, thresholds.spring_forward)

  if(min < offset && offset < max)
    return !dst_is_reversed
  return dst_is_reversed
}

Date.prototype.isDST = function(thresholds) {
  return is_dst(this, thresholds) 
}

is_dst.find_thresholds = find_dst_thresholds

if(typeof module !== 'undefined') {
  module.exports = is_dst
} else {
  window.is_dst = is_dst 
}

})()