Fix Daylight Savings parsing

Previously the offset change for Daylight Savings was unnecessarily
added all the time, resulting in this discrepancy when the local time
was set to UTC-7:

```
Time.parse('2020-11-01 00:00:00')
=> 2020-11-01 00:00:00 -0700
Chronic.parse('2020-11-01 00:00:00')
=> 2020-11-01 01:00:00 -0700
```

Each time we add an offset to a base time, we need to check whether this
crosses Daylight Savings boundaries. Only if it does should we adjust
the time by the offset.

Closes mojombo#147

lib/chronic/repeaters/repeater_time.rb

@@ -81,27 +81,32 @@ def next(pointer)
         yesterday_midnight = midnight - full_day
         tomorrow_midnight = midnight + full_day
 
-        offset_fix = midnight.gmt_offset - tomorrow_midnight.gmt_offset
-        tomorrow_midnight += offset_fix
-
         catch :done do
           if pointer == :future
             if @type.ambiguous?
-              [midnight + @type.time + offset_fix, midnight + half_day + @type.time + offset_fix, tomorrow_midnight + @type.time].each do |t|
+              [midnight, midnight + half_day, tomorrow_midnight].each do |base_time|
+                base_time = adjust_daylight_savings_offset(midnight, base_time)
+                t = adjust_daylight_savings_offset(base_time, base_time + @type.time)
                 (@current_time = t; throw :done) if t >= @now
               end
             else
-              [midnight + @type.time + offset_fix, tomorrow_midnight + @type.time].each do |t|
+              [midnight, tomorrow_midnight].each do |base_time|
+                base_time = adjust_daylight_savings_offset(midnight, base_time)
+                t = adjust_daylight_savings_offset(base_time, base_time + @type.time)
                 (@current_time = t; throw :done) if t >= @now
               end
             end
           else # pointer == :past
             if @type.ambiguous?
-              [midnight + half_day + @type.time + offset_fix, midnight + @type.time + offset_fix, yesterday_midnight + @type.time + half_day].each do |t|
+              [midnight + half_day, midnight, yesterday_midnight + half_day].each do |base_time|
+                base_time = adjust_daylight_savings_offset(midnight, base_time)
+                t = adjust_daylight_savings_offset(base_time, base_time + @type.time)
                 (@current_time = t; throw :done) if t <= @now
               end
             else
-              [midnight + @type.time + offset_fix, yesterday_midnight + @type.time].each do |t|
+              [midnight, yesterday_midnight].each do |base_time|
+                base_time = adjust_daylight_savings_offset(midnight, base_time)
+                t = adjust_daylight_savings_offset(base_time, base_time + @type.time)
                 (@current_time = t; throw :done) if t <= @now
               end
             end
@@ -119,6 +124,26 @@ def next(pointer)
       Span.new(@current_time, @current_time + width)
     end
 
+    # Every time a time crosses a Daylight Savings interval we must adjust the
+    # current time by that amount. For example, if we take midnight of Daylight
+    # Savings and only add an hour, the offset does not change:
+    #
+    # Time.parse('2008-03-09 00:00')
+    # => 2008-03-09 00:00:00 -0800
+    # Time.parse('2008-03-09 00:00') + (60 * 60)
+    # => 2008-03-09 01:00:00 -0800
+    #
+    # However, if we add 2 hours, we notice the time advances to 03:00 instead of 02:00:
+    #
+    # Time.parse('2008-03-09 00:00') + (60 * 60 * 2)
+    # => 2008-03-09 03:00:00 -0700
+    #
+    # Since we gained an hour and we actually want 02:00, we subtract an hour.
+    def adjust_daylight_savings_offset(base_time, current_time)
+      offset_fix = base_time.gmt_offset - current_time.gmt_offset
+      current_time + offset_fix
+    end
+
     def this(context = :future)
       super
 

test/test_daylight_savings.rb

@@ -86,6 +86,11 @@ def test_end_past
     t = Chronic::RepeaterTime.new('1300')
     t.start = @end_daylight_savings
     assert_equal Time.local(2008, 11, 1, 13), t.next(:past).begin
+
+    # unambiguous - resolve to yesterday
+    t = Chronic::RepeaterTime.new('1300')
+    t.start = @end_daylight_savings + 60 * 60 * 24 # Advance one day
+    assert_equal Time.local(2008, 11, 2, 13), t.next(:past).begin
   end
 
   def test_end_future
@@ -114,5 +119,4 @@ def test_end_future
     t.start = @end_daylight_savings
     assert_equal Time.local(2008, 11, 3, 4), t.next(:future).begin
   end
-
 end

test/test_parsing.rb

@@ -43,6 +43,10 @@ def test_handle_generic
 
     assert_nil Chronic.parse("1/1/32.1")
 
+    time = Chronic.parse("2020-11-01 00:00:00")
+    time2 = Time.parse("2020-11-01 00:00:00")
+    assert_in_delta time, time2, 0.000001
+
     time = Chronic.parse("28th", {:guess => :begin})
     assert_equal Time.new(Time.now.year, Time.now.month, 28), time
   end