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

Solved to 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.

src/timezone/index.js

@@ -69,7 +69,7 @@ const timezone = s => {
   if (result.hasDst === false) {
     result.current.offset = summer
     result.current.isDST = false
-  } else if (inSummerTime(s.epoch, result.change.start, result.change.back, summer) === true) {
+  } else if (inSummerTime(s.epoch, result.change.start, result.change.back, summer, winter) === true) {
     result.current.offset = summer
     result.current.isDST = result.hemisphere === 'North' //dst 'on' in winter in north
   } else {

src/timezone/quick.js

@@ -20,7 +20,7 @@ const quickOffset = s => {
     dec = jul - 1
   }
   let split = obj.dst.split('->')
-  let inSummer = isSummer(s.epoch, split[0], split[1], jul)
+  let inSummer = isSummer(s.epoch, split[0], split[1], jul, dec)
   if (inSummer === true) {
     return jul
   }

src/timezone/summerTime.js

@@ -1,29 +1,17 @@
-const zeroPad = require('../fns').zeroPad
+const MSEC_IN_HOUR = 60 * 60 * 1000;
 
-const serialize = d =>
-  zeroPad(d.getMonth() + 1) + '/' + zeroPad(d.getDate()) + ':' + zeroPad(d.getHours())
+const toUtc = (dstChange, offset, year) => {
+  const [month, rest] = dstChange.split('/');
+  const [day, hour] = rest.split(':');
+  return Date.UTC(year, month - 1, day, hour) - offset * MSEC_IN_HOUR;
+};
 
-// a timezone will begin with a specific offset in january
-// then some will switch to something else between november-march
-const shouldChange = (epoch, start, end, defaultOffset) => {
-  //note: this has a cray order-of-operations issue
-  //we can't get the date, without knowing the timezone, and vice-versa
-  //it's possible that we can miss a dst-change by a few hours.
-  let d = new Date(epoch)
-  //(try to mediate this a little?)
-  let bias = d.getTimezoneOffset() || 0
-  let shift = bias + defaultOffset * 60 //in minutes
-  shift = shift * 60 * 1000 //in ms
-  d = new Date(epoch + shift)
+const shouldChange = (epoch, start, end, summerOffset, winterOffset) => {
+  const year = new Date(epoch).getUTCFullYear();
+  const startUtc = toUtc(start, winterOffset, year);
+  const endUtc = toUtc(end, summerOffset, year);
 
-  let current = serialize(d)
-  //eg. is it after ~november?
-  if (current >= start) {
-    //eg. is it before ~march~ too?
-    if (current < end) {
-      return true
-    }
-  }
-  return false
+  return epoch >= startUtc && epoch < endUtc;
 }
+
 module.exports = shouldChange