diff --git a/traffic_prophet/countmatch/growthfactor.py b/traffic_prophet/countmatch/growthfactor.py
index 5ba28ee..e69254f 100644
--- a/traffic_prophet/countmatch/growthfactor.py
+++ b/traffic_prophet/countmatch/growthfactor.py
@@ -54,20 +54,21 @@ def get_aadt(tc):
     def get_wadt_py(tc):
         """Get WADT for all full weeks within permanent years."""
         cdata = tc.data['Daily Count'].loc[tc.perm_years, :].copy()
-        # Overcomplicated groupby using the start of the week, as dt.week
-        # returns the "week ordinal".  See https://stackoverflow.com/a/55890652
-        cdata['Start of Week'] = (
-            cdata['Date'] -
-            cdata['Date'].dt.dayofweek * np.timedelta64(1, 'D'))
-        wadt = (cdata.groupby(['Year', 'Start of Week'])['Daily Count']
-                .agg(['mean', 'count']))
-        # Only keep full weeks.
-        wadt = wadt.loc[wadt['count'] == 7, ('mean',)].reset_index()
-        wadt.columns = ('Year', 'Start of Week', 'WADT')
 
-        start_year = wadt['Year'].min()
-        wadt['Week'] = (wadt['Start of Week'].dt.week.astype(float) +
-                        52. * (wadt['Year'] - start_year))
+        # Corrective measure, as dt.week returns the "week ordinal".  See
+        # https://stackoverflow.com/a/55890652
+        cdata['Week'] = cdata['Date'].dt.week
+        cdata['Month'] = cdata['Date'].dt.month
+        invalid_dates = (((cdata['Week'] == 1) & (cdata['Month'] == 12)) |
+                         (cdata['Week'] == 53))
+        cdata = cdata.loc[~invalid_dates, :]
+
+        wadt = (cdata.groupby(['Year', 'Week'])['Daily Count']
+                .agg(['mean', 'count'])).rename(columns={'mean': 'WADT'})
+
+        # Only keep full weeks.
+        wadt = wadt.loc[wadt['count'] == 7, ('WADT',)].reset_index()
+        wadt['Time'] = wadt['Week'] + 52. * (wadt['Year'] - wadt['Year'].min())
         return wadt
 
     def fit_growth(self, tc):
diff --git a/traffic_prophet/countmatch/tests/test_growthfactor.py b/traffic_prophet/countmatch/tests/test_growthfactor.py
index 176e457..2018f3f 100644
--- a/traffic_prophet/countmatch/tests/test_growthfactor.py
+++ b/traffic_prophet/countmatch/tests/test_growthfactor.py
@@ -56,30 +56,25 @@ def test_get_wadt_py(self, ptc_oneyear, ptc_multiyear):
         wadt_nov29 = (ptc_oneyear.data['Daily Count']
                       .loc[(2010, 333):(2010, 339), 'Daily Count'].mean())
         assert np.isclose(
-            (wadt_oy.loc[wadt_oy['Start of Week'] == '2010-06-14', 'WADT']
-             .values[0]), wadt_jun14)
+            wadt_oy.loc[wadt_oy['Week'] == 24, 'WADT'].values[0], wadt_jun14)
         assert np.isclose(
-            (wadt_oy.loc[wadt_oy['Start of Week'] == '2010-11-29', 'WADT']
-             .values[0]), wadt_nov29)
+            wadt_oy.loc[wadt_oy['Week'] == 48, 'WADT'].values[0], wadt_nov29)
 
         # For multiyear PTC, confirm we can reproduce data frame.
         wadt_my = self.gfb.get_wadt_py(ptc_multiyear)
 
-        ptc_multiyear_dc = ptc_multiyear.data['Daily Count'].loc[
-            ptc_multiyear.perm_years, :].copy()
-        ptc_multiyear_dc['Week'] = (
-            ptc_multiyear_dc['Date'] -
-            (ptc_multiyear_dc['Date'].dt.dayofweek *
-             np.timedelta64(1, 'D'))).dt.week
-
-        wadt_ref = (ptc_multiyear_dc.loc[ptc_multiyear_dc['Week'] < 53, :]
-                    .groupby(['Year', 'Week']).agg(['mean', 'count']))
-        wadt_ref.columns = ('WADT', 'N_days')
-        wadt_ref = wadt_ref.loc[wadt_ref['N_days'] == 7, :].reset_index()
-
-        wadt_ref['Week'] = (wadt_ref['Week'].astype(float) +
-                            52. * (wadt_ref['Year'] - wadt_ref['Year'].min()))
-
-        # Check that all weeks with 7 days are in wadt.
-        assert np.array_equal(wadt_ref['Week'], wadt_my['Week'])
-        assert np.array_equal(wadt_ref['WADT'], wadt_my['WADT'])
+        wadt_apr26_2010 = (ptc_multiyear.data['Daily Count']
+                           .loc[(2010, 116):(2010, 122), :])
+        wadt_my_apr26_2010 = wadt_my.loc[
+            (wadt_my['Year'] == 2010) & (wadt_my['Week'] == 17), :]
+        assert np.allclose(
+            wadt_my_apr26_2010[['WADT', 'Time']].values.ravel(),
+            np.array([wadt_apr26_2010['Daily Count'].mean(), 17.]))
+
+        wadt_oct15_2012 = (ptc_multiyear.data['Daily Count']
+                           .loc[(2012, 289):(2012, 295), :])
+        wadt_my_oct15_2012 = wadt_my.loc[
+            (wadt_my['Year'] == 2012) & (wadt_my['Week'] == 42), :]
+        assert np.allclose(
+            wadt_my_oct15_2012[['WADT', 'Time']].values.ravel(),
+            np.array([wadt_oct15_2012['Daily Count'].mean(), 146.]))