From f6c88d0629ee255c32db32f8de91161fe876577d Mon Sep 17 00:00:00 2001
From: Sami Samhuri <sami@samhuri.net>
Date: Wed, 28 Dec 2016 10:32:08 -0800
Subject: [PATCH] fix dates crossing DST boundaries for UTC, closes #71

---
 Makefile    |  4 ++--
 strftime.js |  9 ++++++++-
 test.js     | 20 ++++++++++++++++----
 3 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/Makefile b/Makefile
index 9a5354d..4a2b985 100644
--- a/Makefile
+++ b/Makefile
@@ -6,10 +6,10 @@ real-minify: strftime.js
 
 test:
 	TZ=America/Vancouver node test.js
-	TZ=CET node test.js
+	TZ=Europe/Amsterdam node test.js
 
 test-minified: minify
 	TZ=America/Vancouver node test.js ./strftime-min.js
-	TZ=CET node test.js ./strftime-min.js
+	TZ=Europe/Amsterdam node test.js ./strftime-min.js
 
 .PHONY: test test-minified
diff --git a/strftime.js b/strftime.js
index 1dfcc4f..0afda93 100644
--- a/strftime.js
+++ b/strftime.js
@@ -183,7 +183,14 @@
                 timestamp = date.getTime();
 
                 if (_useUtcBasedDate) {
-                    date = new Date(date.getTime() + getTimestampToUtcOffsetFor(date) + _customTimezoneOffset);
+                    var utcOffset = getTimestampToUtcOffsetFor(date);
+                    date = new Date(timestamp + utcOffset + _customTimezoneOffset);
+                    // If we've crossed a DST boundary with this calculation we need to
+                    // adjust the new date accordingly or it will be off by an hour in UTC.
+                    if (getTimestampToUtcOffsetFor(date) !== utcOffset) {
+                        var newUTCOffset = getTimestampToUtcOffsetFor(date);
+                        date = new Date(timestamp + newUTCOffset + _customTimezoneOffset);
+                    }
                 }
             }
 
diff --git a/test.js b/test.js
index 17efa56..9fb999c 100755
--- a/test.js
+++ b/test.js
@@ -42,7 +42,7 @@ ok('Exports');
 
 /// time zones
 if (process.env.TZ == 'America/Vancouver') {
-    testTimezone('P[DS]T');
+    testTimezone('America/Vancouver');
     assert.format('%C', '01', '01', new Date(100, 0, 1));
     assert.format('%X', '11:51:45', '18:51:45');
     assert.format('%c', 'Tue Jun 07 11:51:45 2011', 'Tue Jun 07 18:51:45 2011');
@@ -52,10 +52,16 @@ if (process.env.TZ == 'America/Vancouver') {
     assert.format('%U', '12', '13', new Date('2017-03-26 00:00:00 +0000'));
     assert.format('%U', '13', null, new Date('2017-03-27 00:00:00 +0000'));
     assert.format('%U', '13', '14', new Date('2017-04-02 00:00:00 +0000'));
+
+    // Test dates crossing a DST boundary.
+    var dstStart = +new Date('2016-11-06 01:50:00');
+    assert.format('%T', '01:50:00', '08:50:00', new Date(dstStart))
+    assert.format('%T', '01:00:00', '09:00:00', new Date(dstStart + 10 * 60 * 1000))
+
     ok('Time zones (' + process.env.TZ + ')');
 }
-else if (process.env.TZ == 'CET') {
-    testTimezone('CES?T');
+else if (process.env.TZ == 'Europe/Amsterdam') {
+    testTimezone('Europe/Amsterdam');
     assert.format('%C', '01', '00', new Date(100, 0, 1));
     assert.format('%X', '20:51:45', '18:51:45');
     assert.format('%c', 'Tue Jun 07 20:51:45 2011', 'Tue Jun 07 18:51:45 2011');
@@ -65,10 +71,16 @@ else if (process.env.TZ == 'CET') {
     assert.format('%U', '13', null, new Date('2017-03-26 00:00:00 +0000'));
     assert.format('%U', '13', null, new Date('2017-03-27 00:00:00 +0000'));
     assert.format('%U', '14', null, new Date('2017-04-02 00:00:00 +0000'));
+
+    // Test dates crossing a DST boundary.
+    var dstStart = +new Date('2016-10-30 02:50:00');
+    assert.format('%T', '02:50:00', '00:50:00', new Date(dstStart))
+    assert.format('%T', '02:00:00', '01:00:00', new Date(dstStart + 10 * 60 * 1000))
+
     ok('Time zones (' + process.env.TZ + ')');
 }
 else {
-    console.log('(Current timezone has no tests: ' + (process.env.TZ || 'none') + ')');
+    assert(false, '(Current timezone has no tests: ' + (process.env.TZ || 'none') + ')');
 }
 
 /// check all formats in GMT, most coverage