Try fixing the problem

src/web/tracker/forms.py

@@ -4,6 +4,23 @@ from .models import Game, Platform, Purchase, Session
 
 
 class SessionForm(forms.ModelForm):
+    custom_datetime_widget = forms.SplitDateTimeWidget(
+        date_format=("%d-%m-%Y"),
+        time_format=("%H:%M"),
+        date_attrs={"type": "date"},
+        time_attrs={"type": "time"},
+    )
+    timestamp_start = forms.SplitDateTimeField(
+        input_date_formats="['%d-%m-%Y]",
+        input_time_formats="['%H:%M']",
+        widget=custom_datetime_widget,
+    )
+    timestamp_end = forms.SplitDateTimeField(
+        input_date_formats="['%d-%m-%Y]",
+        input_time_formats="['%H:%M']",
+        widget=custom_datetime_widget,
+    )
+
     class Meta:
         model = Session
         fields = [
@@ -13,13 +30,15 @@ class SessionForm(forms.ModelForm):
             "duration_manual",
             "note",
         ]
-        custom_datetime_widget = forms.SplitDateTimeWidget(
+
-            date_attrs={"type": "date"}, time_attrs={"type": "time"}
+        # fields_classes = {
-        )
+        #     "timestamp_start": custom_datetime_field,
-        widgets = {
+        #     "timestamp_end": custom_datetime_field,
-            "timestamp_start": custom_datetime_widget,
+        # }
-            "timestamp_end": custom_datetime_widget,
+        # widgets = {
-        }
+        #     "timestamp_start": custom_datetime_widget,
+        #     "timestamp_end": custom_datetime_widget,
+        # }
 
 
 class PurchaseForm(forms.ModelForm):

src/web/tracker/views.py

@@ -23,7 +23,11 @@ def add_session(request):
     context = {}
     now = now_with_tz()
     last = Session.objects.all().last()
-    initial = {"timestamp_start": now, "purchase": last.purchase}
+    initial = {
+        "timestamp_start_0": now.date(),
+        "timestamp_start_1": now.time(),
+        "purchase": last.purchase,
+    }
     form = SessionForm(request.POST or None, initial=initial)
     if form.is_valid():
         form.save()