Fix time-only DateTime.parse under freeze (#440)

* Fix time-only DateTime.parse under freeze

DateTime.parse('HH:MM') previously fell back to parsed_date + travel_offset_days,
with travel_offset_days = (@travel_offset / 86400).round. Because @travel_offset
measures the gap between the frozen moment and the real system clock, that rounding
stepped up or down depending on how close the real clock was to a 12‑hour boundary.
For example, freezing at 2017-08-10 10:00 UTC while the host clock is
2025-11-13 22:00 UTC yields an offset of N.5 days, so rounding adds an
extra day and parsing “01:00” returns 2017-08-09 instead of 2017-08-10.

Anchor hour/minute-only parses to the mocked year/month/day and add deterministic
regression tests to ensure time-only input no longer depends on the host clock.

* Rename tests for DateTime.parse HH:MM format

- test_date_time_parse_hhmm_uses_frozen_date_not_real_clock: regression test ensuring parsed time uses frozen date, not host clock
- test_date_time_parse_hhmm_format_returns_correct_time: basic HH:MM format parsing

* update test according to the PR comments konieczkow made

---------

Co-authored-by: Josh Cronemeyer <joshuacronemeyer@gmail.com>

Author: konieczkow

lib/timecop/time_extensions.rb

@@ -154,6 +154,8 @@ def parse_with_mock_date(*args)
         Date.closest_wday(date_hash[:wday]).to_datetime
       when date_hash[:hour] && date_hash[:min] && date_hash[:sec]
         DateTime.new(mocked_time_stack_item.year, mocked_time_stack_item.month, mocked_time_stack_item.day, date_hash[:hour], date_hash[:min], date_hash[:sec])
+      when date_hash[:hour] && date_hash[:min]
+        DateTime.new(mocked_time_stack_item.year, mocked_time_stack_item.month, mocked_time_stack_item.day, date_hash[:hour], date_hash[:min], 0)
       else
         parsed_date + mocked_time_stack_item.travel_offset_days
       end

test/date_time_parse_scenarios.rb

@@ -64,6 +64,24 @@ def test_date_time_parse_time_only_scenario
     assert_equal DateTime.parse("2008-09-01T15:00:00"), DateTime.parse('15:00:00')
   end
 
+  def test_date_time_parse_hhmm_uses_frozen_date_not_real_clock
+    real_now = Time.now_without_mock_time.utc
+    offset = 12 * 3600 + 30
+    freeze_time = real_now.hour >= 12 ? real_now - offset : real_now + offset
+
+    Timecop.freeze(freeze_time) do
+      expected = DateTime.new(freeze_time.year, freeze_time.month, freeze_time.day, 1, 0, 0, 0)
+      assert_equal expected, DateTime.parse('01:00')
+    end
+  end
+
+  def test_date_time_parse_hhmm_format_returns_correct_time
+    assert_equal DateTime.new(2008, 9, 1, 0, 0, 0), DateTime.parse('00:00')
+    assert_equal DateTime.new(2008, 9, 1, 1, 0, 0), DateTime.parse('01:00')
+    assert_equal DateTime.new(2008, 9, 1, 12, 30, 0), DateTime.parse('12:30')
+    assert_equal DateTime.new(2008, 9, 1, 23, 59, 0), DateTime.parse('23:59')
+  end
+
   def test_date_time_parse_month_year
     assert_equal DateTime.parse("2012-12-01"), DateTime.parse('DEC 2012')
   end