diff --git a/coffeebuddy/model.py b/coffeebuddy/model.py
index e2659a2..92906ce 100644
--- a/coffeebuddy/model.py
+++ b/coffeebuddy/model.py
@@ -17,6 +17,13 @@ def db_weekday(column) -> str:
         return sqlalchemy.func.strftime("%w", column)
 
 
+def db_date_format(column):
+    if flask.current_app.db.engine.name == "postgresql":
+        return sqlalchemy.func.to_char(column, "YYYY-MM-DD")
+    else:
+        return sqlalchemy.func.strftime("%Y-%m-%d", column)
+
+
 def weekday(number):
     """
     Helper to return the name of the weekday for given day number.
@@ -263,15 +270,15 @@ def drinks_last_weeks(
                 zip(
                     *db.session.execute(
                         db.select(
-                            db.func.Date(Drink.timestamp).label("day"),
+                            db_date_format(db.func.Date(Drink.timestamp)),
                             db.func.count(db.func.Date(Drink.timestamp)),
                         )
                         .where(
                             (Drink.userid == self.id)
                             & (db.func.Date(Drink.timestamp) >= since)
                         )
-                        .group_by("day")
-                        .order_by("day")
+                        .group_by(db.func.Date(Drink.timestamp))
+                        .order_by(db.func.Date(Drink.timestamp))
                     ).all()
                 )
             )
@@ -310,7 +317,7 @@ def drinks_last_weeks_all(since=timedelta(weeks=12)) -> Tuple[List[str], List[in
 
     def drinks_avg_today(self) -> float:
         db = flask.current_app.db
-        today_weekday = str(date.today().weekday())
+        today_weekday = str(date.today().isoweekday() % 7)
 
         data = db.session.scalars(
             db.select(db.func.count("*"))
diff --git a/tests/test_database.py b/tests/test_database.py
index 7431119..fa96378 100644
--- a/tests/test_database.py
+++ b/tests/test_database.py
@@ -1,3 +1,5 @@
+from datetime import datetime, timedelta
+
 import sqlalchemy.exc
 
 from . import TestCoffeebuddy
@@ -159,6 +161,35 @@ def test_by_tag_none_fails(self):
         self.assertIsNone(User.by_tag(None))
         self.assertIsNone(User.by_tag(b""))
 
+    def test_drinks_last_weeks(self):
+        from coffeebuddy.model import Drink
+
+        user1, _ = self.add_default_user()
+        user1.drinks.append(Drink(price=1))
+        self.db.session.commit()
+        self.assertEqual(
+            user1.drinks_last_weeks(group_by="day"),
+            ((datetime.now().strftime("%Y-%m-%d"),), (1,)),
+        )
+
+    def test_drinks_avg_today(self):
+        from coffeebuddy.model import Drink
+
+        user1, _ = self.add_default_user()
+        user1.drinks.append(Drink(price=1))
+        self.db.session.commit()
+        self.assertEqual(user1.drinks_avg_today(), 1.0)
+        user1.drinks.extend(
+            (
+                Drink(price=1, timestamp=datetime.now() - timedelta(weeks=1)),
+                Drink(price=1, timestamp=datetime.now() - timedelta(weeks=1)),
+                Drink(price=1, timestamp=datetime.now() - timedelta(weeks=2)),
+                Drink(price=1, timestamp=datetime.now() - timedelta(weeks=3)),
+            )
+        )
+        self.db.session.commit()
+        self.assertEqual(user1.drinks_avg_today(), 1.25)
+
 
 class TestModelDrink(TestCoffeebuddy):
     def test_add_drink(self):