Fixes for FasterXML#1661

In summary, the custom DateFormat always has its TimeZone forced to whatever `Settings.getTimeZone()` will return.
Provide an additional fix in `withTimeZone` to handle the case where `_dateFormat` is null (since this is allowed by `withDateFormat`).
Update test case to reflect the fixed behavior.

src/main/java/com/fasterxml/jackson/databind/cfg/BaseSettings.java

@@ -238,14 +238,24 @@ public BaseSettings withTypeResolverBuilder(TypeResolverBuilder<?> typer) {
                 _timeZone, _defaultBase64);
     }
 
+    /**
+     * Set the {@link DateFormat} instance to use with this settings.
+     * <p>
+     * Note that the TimeZone value originally set on the DateFormat will be ignored and
+     * reset reset to use the value configured on this settings object.
+     * The locale will remains unchanged however.
+     * 
+     * @param df the {@link DateFormat} instance to use
+     * @return a new instance of the settings with the DateFormat set to the desired value
+     */
     public BaseSettings withDateFormat(DateFormat df) {
         if (_dateFormat == df) {
             return this;
         }
-        // 26-Sep-2015, tatu: Related to [databind#939], let's try to force TimeZone if
-        //   (but only if!) it has been set explicitly.
-        if ((df != null) && hasExplicitTimeZone()) {
-            df = _force(df, _timeZone);
+
+        // Configure the DateFormat with the TimeZone mandated by this settings
+        if (df != null ) {
+            df = _force(df, getTimeZone());
         }
         return new BaseSettings(_classIntrospector, _annotationIntrospector, _visibilityChecker, _propertyNamingStrategy, _typeFactory,
                 _typeResolverBuilder, df, _handlerInstantiator, _locale,
@@ -284,7 +294,11 @@ public BaseSettings with(TimeZone tz)
             return this;
         }
 
-        DateFormat df = _force(_dateFormat, tz);
+        // Reconfigure the DateFormat with the new TimeZone
+        DateFormat df = _dateFormat;
+        if( df != null ) {
+        	df = _force(_dateFormat, tz);
+        }
         return new BaseSettings(_classIntrospector, _annotationIntrospector,
                 _visibilityChecker, _propertyNamingStrategy, _typeFactory,
                 _typeResolverBuilder, df, _handlerInstantiator, _locale,
@@ -346,16 +360,27 @@ public Locale getLocale() {
         return _locale;
     }
 
+    /**
+     * Get the {@link TimeZone} explicitly set on this settings object or the 
+     * default value otherwise.
+     * 
+     * @return the {@link TimeZone} to use according to this settings object (not null).
+     * 
+     * @see #with(TimeZone)
+     * @see #hasExplicitTimeZone()
+     * @see #DEFAULT_TIMEZONE
+     */
     public TimeZone getTimeZone() {
         TimeZone tz = _timeZone;
         return (tz == null) ? DEFAULT_TIMEZONE : tz;
     }
 
     /**
      * Accessor that may be called to determine whether this settings object
-     * has been explicitly configured with a TimeZone (true), or is still
-     * relying on the default settings (false).
+     * has been explicitly configured with a TimeZone {@code true}, or is still
+     * relying on the default settings {@code false}.
      *
+     * @return {@code true} if a TimeZone has been explicitly configured
      * @since 2.7
      */
     public boolean hasExplicitTimeZone() {
@@ -372,6 +397,13 @@ public Base64Variant getBase64Variant() {
     /**********************************************************
      */
 
+    /**
+     * Set the timeZone on the given DateFormat
+     * 
+     * @param df the DateFormat instance to configure
+     * @param tz the TimeZone to set on the DateFormat
+     * @return a potentially new instance of the DateFormat with the TimeZone set to the desired value
+     */
     private DateFormat _force(DateFormat df, TimeZone tz)
     {
         if (df instanceof StdDateFormat) {

src/test/java/com/fasterxml/jackson/databind/deser/DateDeserializationTest.java

@@ -439,23 +439,28 @@ public void testDateUtil_Annotation_TimeZone() throws Exception
      */
     public void testDateUtil_customDateFormat_withoutTZ() throws Exception
     {
-        // FIXME
-        //
-        // The general rule with the StdDateFormat is:
-        //     the TimeZone of the ObjectMapper is used if the JSON doesn't hold
-        //     any timezone/offset information.
+        // The general rule is:
+        //     the TimeZone of the ObjectMapper is used when parsing dates if the JSON 
+        //     doesn't contain any timezone/offset information.
         //
         // This rule remains valid with the @JsonFormat annotation unless it forces
-        // an explicit timezeone, in which case the latter takes precedence.
-        //
-        // One would expect the same behavior when the StdDateFormat is replaced by a 
-        // custom DateFormat on the ObjectMapper. In other words, the timezone of the 
-        // DateFormat is of no importance: the ObjectMapper's default should be used
-        // whenever it is needed.
+        // an explicit timezone, in which case the latter takes precedence.
+
 
+        // Test first with a vanilla ObjectMapper (defaults to UTC)
+        {
+            DateFormat df = new SimpleDateFormat("yyyy-MM-dd'X'HH:mm:ss");
+            df.setTimeZone( TimeZone.getTimeZone("GMT+4") );    // TZ different from mapper's default
+
+            ObjectMapper mapper = new ObjectMapper();
+            mapper.setDateFormat(df);
+            // Note it is important NOT TO CALL mapper.setTimeZone(...) in this test..
+
+            // The mapper's default TZ is used...
+            verify(mapper, "2000-01-02X04:00:00", judate(2000, 1, 2, 4, 00, 00, 00, "UTC"));
+        }
 
-        // Test first with a non default TZ on the ObjectMapper
-        // --> OK: the mapper's default TZ is used to parse the date.
+        // Test with a non default TZ on the ObjectMapper
         {
             DateFormat df = new SimpleDateFormat("yyyy-MM-dd'X'HH:mm:ss");
             df.setTimeZone( TimeZone.getTimeZone("GMT+4") );    // TZ different from mapper's default
@@ -468,19 +473,18 @@ public void testDateUtil_customDateFormat_withoutTZ() throws Exception
             verify(mapper, "2000-01-02X04:00:00", judate(2000, 1, 2, 4, 00, 00, 00, LOCAL_TZ));
         }
 
-        // Test a second time with the default TZ on the ObjectMapper
-        // Note it is important NOT TO CALL mapper.setTimeZone(...) in this test..
-        // --> KO: the custom format's TZ is used instead of the mapper's default as above.
-        //
+        // Same test as above but set the DateFormat before the TimeZone (make sure order of 
+        // settings doesn't matter)
         {
             DateFormat df = new SimpleDateFormat("yyyy-MM-dd'X'HH:mm:ss");
             df.setTimeZone( TimeZone.getTimeZone("GMT+4") );    // TZ different from mapper's default
 
             ObjectMapper mapper = new ObjectMapper();
             mapper.setDateFormat(df);
+            mapper.setTimeZone( TimeZone.getTimeZone(LOCAL_TZ) );
 
-            // FIXME mapper's default TZ should have been used
-            verify(mapper, "2000-01-02X04:00:00", judate(2000, 1, 2, 4, 00, 00, 00, "GMT+4"));
+            // The mapper's default TZ is used...
+            verify(mapper, "2000-01-02X04:00:00", judate(2000, 1, 2, 4, 00, 00, 00, LOCAL_TZ));
         }
     }