diff --git a/src/bokeh/util/serialization.py b/src/bokeh/util/serialization.py
index a64a504d761..16029e5df6b 100644
--- a/src/bokeh/util/serialization.py
+++ b/src/bokeh/util/serialization.py
@@ -144,7 +144,11 @@ def convert_date_to_datetime(obj: dt.date) -> float:
         datetime
 
     '''
-    return (dt.datetime(*obj.timetuple()[:6], tzinfo=dt.timezone.utc) - DT_EPOCH).total_seconds() * 1000
+    if isinstance(obj, dt.datetime):
+        dt_obj = obj.replace(tzinfo=dt.timezone.utc)
+    else:
+        dt_obj = dt.datetime(obj.year, obj.month, obj.day, tzinfo=dt.timezone.utc)
+    return (dt_obj - DT_EPOCH).total_seconds() * 1000
 
 def convert_timedelta_type(obj: dt.timedelta | np.timedelta64) -> float:
     ''' Convert any recognized timedelta value to floating point absolute