From 914c7c791bea621cfc7ea7c891a6edc993264d43 Mon Sep 17 00:00:00 2001
From: Alan Liu <alansliu@gmail.com>
Date: Fri, 9 May 2003 18:44:54 +0000
Subject: [PATCH] ICU-2792 fix anomalous behavior when adding across a DST
 boundary

X-SVN-Rev: 11864
---
 .../dev/test/calendar/IBMCalendarTest.java    | 40 ++++++++++++++++++-
 icu4j/src/com/ibm/icu/util/Calendar.java      | 13 ++++++
 2 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/icu4j/src/com/ibm/icu/dev/test/calendar/IBMCalendarTest.java b/icu4j/src/com/ibm/icu/dev/test/calendar/IBMCalendarTest.java
index 778a3334080..2fc0c3c5cf4 100755
--- a/icu4j/src/com/ibm/icu/dev/test/calendar/IBMCalendarTest.java
+++ b/icu4j/src/com/ibm/icu/dev/test/calendar/IBMCalendarTest.java
@@ -4,8 +4,8 @@
  * others. All Rights Reserved.
  *******************************************************************************
  * $Source: /xsrl/Nsvn/icu/icu4j/src/com/ibm/icu/dev/test/calendar/IBMCalendarTest.java,v $ 
- * $Date: 2003/04/04 17:15:13 $ 
- * $Revision: 1.15 $
+ * $Date: 2003/05/09 18:44:53 $ 
+ * $Revision: 1.16 $
  *******************************************************************************
  */
 package com.ibm.icu.dev.test.calendar;
@@ -411,6 +411,42 @@ public class IBMCalendarTest extends CalendarTest {
         }
     }
 
+    /**
+     * Make sure that when adding a day, we actually wind up in a
+     * different day.  The DST adjustments we use to keep the hour
+     * constant across DST changes can backfire and change the day.
+     */
+    public void TestTimeZoneTransitionAdd() {
+        Locale locale = Locale.US; // could also be CHINA
+        SimpleDateFormat dateFormat =
+            new SimpleDateFormat("MM/dd/yyyy HH:mm z", locale);
+
+        String tz[] = TimeZone.getAvailableIDs();
+
+        for (int z=0; z<tz.length; ++z) {
+            TimeZone t = TimeZone.getTimeZone(tz[z]);
+            dateFormat.setTimeZone(t);
+
+            Calendar cal = Calendar.getInstance(t, locale);
+            cal.clear();
+            // Scan the year 2003, overlapping the edges of the year
+            cal.set(Calendar.YEAR, 2002);
+            cal.set(Calendar.MONTH, Calendar.DECEMBER);
+            cal.set(Calendar.DAY_OF_MONTH, 25);
+
+            for (int i=0; i<365+10; ++i) {
+                Date yesterday = cal.getTime();
+                int yesterday_day = cal.get(Calendar.DAY_OF_MONTH);
+                cal.add(Calendar.DAY_OF_MONTH, 1);
+                if (yesterday_day == cal.get(Calendar.DAY_OF_MONTH)) {
+                    errln(tz[z] + " " +
+                          dateFormat.format(yesterday) + " +1d= " +
+                          dateFormat.format(cal.getTime()));
+            }
+        }
+    }
+}
+
     /**
      * Miscellaneous tests to increase coverage.
      */
diff --git a/icu4j/src/com/ibm/icu/util/Calendar.java b/icu4j/src/com/ibm/icu/util/Calendar.java
index de5f3407d63..a2b0bda1e20 100755
--- a/icu4j/src/com/ibm/icu/util/Calendar.java
+++ b/icu4j/src/com/ibm/icu/util/Calendar.java
@@ -2767,8 +2767,10 @@ public abstract class Calendar implements Serializable, Cloneable {
         // operation.  If it has changed, then adjust the millis to
         // compensate.
         int dst = 0;
+        int hour = 0;
         if (keepHourInvariant) {
             dst = get(DST_OFFSET);
+            hour = internalGet(HOUR_OF_DAY);
         }
 
         setTimeInMillis(getTimeInMillis() + delta);
@@ -2776,7 +2778,18 @@ public abstract class Calendar implements Serializable, Cloneable {
         if (keepHourInvariant) {
             dst -= get(DST_OFFSET);
             if (dst != 0) {
+                // We have done an hour-invariant adjustment but the
+                // DST offset has altered.  We adjust millis to keep
+                // the hour constant.  In cases such as midnight after
+                // a DST change which occurs at midnight, there is the
+                // danger of adjusting into a different day.  To avoid
+                // this we make the adjustment only if it actually
+                // maintains the hour.
+                long t = time;
                 setTimeInMillis(time + dst);
+                if (get(HOUR_OF_DAY) != hour) {
+                    setTimeInMillis(t);
+                }
             }
         }
     }