diff --git a/python/lsst/daf/butler/dimensions/_records.py b/python/lsst/daf/butler/dimensions/_records.py
index 60e52ea0a8..fd4614c0f1 100644
--- a/python/lsst/daf/butler/dimensions/_records.py
+++ b/python/lsst/daf/butler/dimensions/_records.py
@@ -296,6 +296,8 @@ def __init__(self, **kwargs: Any):
             # Note that we remove from kwargs as we go, to make sure there's
             # nothing left at the end.
             object.__setattr__(self, name, kwargs.pop(name, None))
+        # Support 'datetime_begin' and 'datetime_end' instead of 'timespan' for
+        # backwards compatibility, but if one is provided both must be.
         if self.definition.temporal is not None and self.timespan is None and "datetime_begin" in kwargs:
             object.__setattr__(
                 self,