Remediate the 2026-09-03 feature-test findings

shopdb/core/models/base.py

@@ -1,6 +1,6 @@
 """Base model class with common fields."""
 
-from datetime import datetime, timezone
+from datetime import date, datetime, timezone
 from shopdb.extensions import db
 
 
@@ -63,6 +63,13 @@ class BaseModel(db.Model):
             value = getattr(self, c.name)
             if isinstance(value, datetime):
                 value = value.isoformat() + 'Z'
+            elif isinstance(value, date):
+                # A date, not an instant: no time and no timezone marker. Left
+                # to Flask's encoder it became an HTTP-date
+                # ("Thu, 15 Jan 2026 00:00:00 GMT"), which a browser parses
+                # back into a moment and can render as the day before.
+                # datetime is a subclass of date, so this must stay second.
+                value = value.isoformat()
             result[c.name] = value
         return result