WIP: Use approximate comparison when checking for changes to start dates

cms/templates/edit_subsection.html

 <%inherit file="base.html" />
 <%!
    import logging
-   from xmodule.util.date_utils import get_default_time_display
+   from xmodule.util.date_utils import get_default_time_display, almost_same_datetime
 %>
 
 <%! from django.core.urlresolvers import reverse %>
@@ -47,9 +47,10 @@
                 placeholder="HH:MM" class="time" size='10' autocomplete="off"/>
           </div>
         </div>
-        % if subsection.lms.start != parent_item.lms.start and subsection.lms.start:
+        % if subsection.lms.start and not almost_same_datetime(subsection.lms.start, parent_item.lms.start):
             % if parent_item.lms.start is None:
-            <p class="notice">The date above differs from the release date of ${parent_item.display_name_with_default}, which is unset.
+            <p class="notice">The date above differs from the release date of 
+            ${parent_item.display_name_with_default}, which is unset.
             % else:
             <p class="notice">The date above differs from the release date of ${parent_item.display_name_with_default} –
                 ${get_default_time_display(parent_item.lms.start)}.

common/lib/xmodule/xmodule/tests/test_date_utils.py

 # Tests for xmodule.util.date_utils
 
-from nose.tools import assert_equals
-from xmodule.util import date_utils
-import datetime
+from nose.tools import assert_equals, assert_false
+from xmodule.util.date_utils import get_default_time_display, almost_same_datetime
+from datetime import datetime, timedelta, tzinfo
 from pytz import UTC
 
 
 def test_get_default_time_display():
-    assert_equals("", date_utils.get_default_time_display(None))
-    test_time = datetime.datetime(1992, 3, 12, 15, 3, 30, tzinfo=UTC)
+    assert_equals("", get_default_time_display(None))
+    test_time = datetime(1992, 3, 12, 15, 3, 30, tzinfo=UTC)
     assert_equals(
         "Mar 12, 1992 at 15:03 UTC",
-        date_utils.get_default_time_display(test_time))
+        get_default_time_display(test_time))
     assert_equals(
         "Mar 12, 1992 at 15:03 UTC",
-        date_utils.get_default_time_display(test_time, True))
+        get_default_time_display(test_time, True))
     assert_equals(
         "Mar 12, 1992 at 15:03",
-        date_utils.get_default_time_display(test_time, False))
+        get_default_time_display(test_time, False))
 
 def test_get_default_time_display_notz():
-    test_time = datetime.datetime(1992, 3, 12, 15, 3, 30)
+    test_time = datetime(1992, 3, 12, 15, 3, 30)
     assert_equals(
         "Mar 12, 1992 at 15:03 UTC",
-        date_utils.get_default_time_display(test_time))
+        get_default_time_display(test_time))
     assert_equals(
         "Mar 12, 1992 at 15:03 UTC",
-        date_utils.get_default_time_display(test_time, True))
+        get_default_time_display(test_time, True))
     assert_equals(
         "Mar 12, 1992 at 15:03",
-        date_utils.get_default_time_display(test_time, False))
+        get_default_time_display(test_time, False))
 
 # pylint: disable=W0232
-class NamelessTZ(datetime.tzinfo):
+class NamelessTZ(tzinfo):
 
     def utcoffset(self, _dt):
-        return datetime.timedelta(hours=-3)
+        return timedelta(hours=-3)
 
     def dst(self, _dt):
-        return datetime.timedelta(0)
+        return timedelta(0)
 
 def test_get_default_time_display_no_tzname():
-    assert_equals("", date_utils.get_default_time_display(None))
-    test_time = datetime.datetime(1992, 3, 12, 15, 3, 30, tzinfo=NamelessTZ())
+    assert_equals("", get_default_time_display(None))
+    test_time = datetime(1992, 3, 12, 15, 3, 30, tzinfo=NamelessTZ())
     assert_equals(
         "Mar 12, 1992 at 15:03-0300",
-        date_utils.get_default_time_display(test_time))
+        get_default_time_display(test_time))
     assert_equals(
         "Mar 12, 1992 at 15:03-0300",
-        date_utils.get_default_time_display(test_time, True))
+        get_default_time_display(test_time, True))
     assert_equals(
         "Mar 12, 1992 at 15:03",
-        date_utils.get_default_time_display(test_time, False))
+        get_default_time_display(test_time, False))
+
+def test_almost_same_datetime():
+    assert almost_same_datetime(
+        datetime(2013, 5, 3, 10, 20, 30),
+        datetime(2013, 5, 3, 10, 21, 29)
+    )
+
+    assert almost_same_datetime(
+        datetime(2013, 5, 3, 11, 20, 30),
+        datetime(2013, 5, 3, 10, 21, 29),
+        timedelta(hours=1)
+    )
+
+    assert_false(
+        almost_same_datetime(
+            datetime(2013, 5, 3, 11, 20, 30),
+            datetime(2013, 5, 3, 10, 21, 29)
+        )
+    )
+
+    assert_false(
+        almost_same_datetime(
+            datetime(2013, 5, 3, 11, 20, 30),
+            datetime(2013, 5, 3, 10, 21, 29),
+            timedelta(minutes=10)
+        )
+    )

common/lib/xmodule/xmodule/tests/test_export.py

@@ -185,6 +185,9 @@ class TestEdxJsonEncoder(unittest.TestCase):
             self.encoder.default(datetime(2013, 5, 3, 10, 20, 30, 0, self.null_utc_tz))
         )
 
-    def test_other_classes(self):
+    def test_fallthrough(self):
         with self.assertRaises(TypeError):
             self.encoder.default(None)
+
+        with self.assertRaises(TypeError):
+            self.encoder.default({})

common/lib/xmodule/xmodule/util/date_utils.py

+import datetime
 def get_default_time_display(dt, show_timezone=True):
     """
     Converts a datetime to a string representation. This is the default
@@ -20,3 +21,14 @@ def get_default_time_display(dt, show_timezone=True):
         else:
             timezone = " UTC"
     return dt.strftime("%b %d, %Y at %H:%M") + timezone
+
+
+def almost_same_datetime(dt1, dt2, allowed_delta=datetime.timedelta(minutes=1)):
+    """
+    Returns true if these are w/in a minute of each other. (in case secs saved to db
+    or timezone aren't same)
+
+    :param dt1:
+    :param dt2:
+    """
+    return abs(dt1 - dt2) < allowed_delta