Fix test failure for timezones west of the prime meridian.

The previous fix didn't calculate the comparison time correctly and
ended up in the wrong day because the GMT time is before the local time.

In the course of checking the corner-case timezones (Midway and
Kiritimati) I found an error in the GncDateTime calculation of the
neutral time, so fixed that too.

bindings/python/tests/test_business.py

@@ -1,6 +1,6 @@
 from unittest import main
 
-from datetime import datetime, timezone
+from datetime import datetime, timezone, timedelta
 
 from gnucash import Account, \
     ACCT_TYPE_RECEIVABLE, ACCT_TYPE_INCOME, ACCT_TYPE_BANK, \
@@ -56,8 +56,15 @@ class TestBusiness(BusinessSession):
         self.assertEqual( NAME, self.employee.GetUsername() )
 
     def test_post(self):
-        self.assertEqual(datetime.now(timezone.utc).replace(hour=10, minute=59, second=0, microsecond=0).astimezone(),
-                         self.invoice.GetDatePosted().astimezone())
+        utc_offset = datetime.now().astimezone().utcoffset()
+        now = datetime.now().astimezone()
+        neutral_time = (now + utc_offset).astimezone(timezone.utc).replace(hour=10, minute=59, second=0, microsecond=0)
+        if utc_offset > timedelta(hours=13):
+            neutral_time -= utc_offset - timedelta(hours=13);
+        if utc_offset < timedelta(hours=-10):
+            neutral_time += timedelta(hours=-10) - utc_offset
+        self.assertEqual(neutral_time,
+                         self.invoice.GetDatePosted().astimezone(timezone.utc))
         self.assertTrue( self.invoice.IsPosted() )
 
     def test_owner(self):

libgnucash/engine/gnc-datetime.cpp

@@ -314,7 +314,7 @@ GncDateTimeImpl::GncDateTimeImpl(const GncDateImpl& date, DayPart part) :
             if (offset < hours(-10))
                 m_time -= hours(offset.hours() + 10);
             if (offset > hours(13))
-                m_time -= hours(offset.hours() - 11);
+                m_time += hours(13 - offset.hours());
         }
         catch(boost::gregorian::bad_year&)
         {