diff --git a/agenda/__init__.py b/agenda/__init__.py
index 86c755b..b44ceb2 100644
--- a/agenda/__init__.py
+++ b/agenda/__init__.py
@@ -328,6 +328,7 @@ def critical_mass(start_date: date, limit: int = 12) -> list[Event]:
 
 
 def as_date(d: date | datetime) -> date:
+    """Return date for given date or datetime."""
     return d.date() if isinstance(d, datetime) else d