Backport DST fix from v2 (#1005)

See #1003

CHANGELOG.md

@@ -10,6 +10,12 @@ This change log follows the format documented in [Keep a CHANGELOG].
 
 ## [Unreleased]
 
+### Fixed
+
+- [Fixed DST issue](https://github.com/date-fns/date-fns/pull/1005). See [#972](https://github.com/date-fns/date-fns/issues/972) and [#992](https://github.com/date-fns/date-fns/issues/992) for more details. This fix was backported from v2.
+
+- Fix a few bugs that appear in timezones with offsets that include seconds (e.g. GMT+00:57:44). See PR [#789](https://github.com/date-fns/date-fns/issues/789). This fix was backported from v2.
+
 ## [1.29.0] - 2017-10-11
 
 ### Fixed

src/_lib/getTimezoneOffsetInMilliseconds/index.js

@@ -0,0 +1,21 @@
+var MILLISECONDS_IN_MINUTE = 60000
+
+/**
+ * Google Chrome as of 67.0.3396.87 introduced timezones with offset that includes seconds.
+ * They usually appear for dates that denote time before the timezones were introduced
+ * (e.g. for 'Europe/Prague' timezone the offset is GMT+00:57:44 before 1 October 1891
+ * and GMT+01:00:00 after that date)
+ *
+ * Date#getTimezoneOffset returns the offset in minutes and would return 57 for the example above,
+ * which would lead to incorrect calculations.
+ *
+ * This function returns the timezone offset in milliseconds that takes seconds in account.
+ */
+module.exports = function getTimezoneOffsetInMilliseconds (dirtyDate) {
+  var date = new Date(dirtyDate.getTime())
+  var baseTimezoneOffset = date.getTimezoneOffset()
+  date.setSeconds(0, 0)
+  var millisecondsPartOfTimezoneOffset = date.getTime() % MILLISECONDS_IN_MINUTE
+
+  return baseTimezoneOffset * MILLISECONDS_IN_MINUTE + millisecondsPartOfTimezoneOffset
+}

src/parse/index.js

@@ -1,3 +1,4 @@
+var getTimezoneOffsetInMilliseconds = require('../_lib/getTimezoneOffsetInMilliseconds/index.js')
 var isDate = require('../is_date/index.js')
 
 var MILLISECONDS_IN_HOUR = 3600000
@@ -107,14 +108,25 @@ function parse (argument, dirtyOptions) {
     }
 
     if (dateStrings.timezone) {
-      offset = parseTimezone(dateStrings.timezone)
+      offset = parseTimezone(dateStrings.timezone) * MILLISECONDS_IN_MINUTE
     } else {
-      // get offset accurate to hour in timezones that change offset
-      offset = new Date(timestamp + time).getTimezoneOffset()
-      offset = new Date(timestamp + time + offset * MILLISECONDS_IN_MINUTE).getTimezoneOffset()
+      var fullTime = timestamp + time
+      var fullTimeDate = new Date(fullTime)
+
+      offset = getTimezoneOffsetInMilliseconds(fullTimeDate)
+
+      // Adjust time when it's coming from DST
+      var fullTimeDateNextDay = new Date(fullTime)
+      fullTimeDateNextDay.setDate(fullTimeDate.getDate() + 1)
+      var offsetDiff =
+        getTimezoneOffsetInMilliseconds(fullTimeDateNextDay) -
+        getTimezoneOffsetInMilliseconds(fullTimeDate)
+      if (offsetDiff > 0) {
+        offset += offsetDiff
+      }
     }
 
-    return new Date(timestamp + time + offset * MILLISECONDS_IN_MINUTE)
+    return new Date(timestamp + time + offset)
   } else {
     return new Date(argument)
   }