diff --git a/src/bokeh/core/property/datetime.py b/src/bokeh/core/property/datetime.py
index 8198d3ca0ad..fee8995724c 100644
--- a/src/bokeh/core/property/datetime.py
+++ b/src/bokeh/core/property/datetime.py
@@ -14,6 +14,10 @@
 from __future__ import annotations
 
 import logging # isort:skip
+from bokeh.core.property.bases import Init, Property
+from bokeh.core.property.singletons import Undefined
+from bokeh.util.serialization import convert_date_to_datetime
+
 log = logging.getLogger(__name__)
 
 #-----------------------------------------------------------------------------
@@ -89,12 +93,23 @@ def __init__(self, default: Init[str | datetime.date | datetime.datetime] = Unde
     def transform(self, value: Any) -> Any:
         value = super().transform(value)
 
+
+        # Fast path: already a datetime.datetime
+        if isinstance(value, datetime.datetime):
+            # UTC-ize if needed (handled by convert_date_to_datetime)
+            return convert_date_to_datetime(value)
+
+        # String to datetime conversion
         if isinstance(value, str):
             value = datetime.datetime.fromisoformat(value)
 
         # Handled by serialization in protocol.py for now, except for Date
+            return convert_date_to_datetime(value)
+
+        # datetime.date but not datetime.datetime
         if isinstance(value, datetime.date):
-            value = convert_date_to_datetime(value)
+            return convert_date_to_datetime(value)
+
 
         return value
 
diff --git a/src/bokeh/util/serialization.py b/src/bokeh/util/serialization.py
index a64a504d761..3ccd867d641 100644
--- a/src/bokeh/util/serialization.py
+++ b/src/bokeh/util/serialization.py
@@ -84,7 +84,7 @@ def __getattr__(name: str) -> Any:
 NP_EPOCH = np.datetime64(0, 'ms')
 NP_MS_DELTA = np.timedelta64(1, 'ms')
 
-DT_EPOCH = dt.datetime.fromtimestamp(0, tz=dt.timezone.utc)
+DT_EPOCH = dt.datetime(1970, 1, 1, tzinfo=dt.timezone.utc)
 
 __doc__ = format_docstring(__doc__, binary_array_types="\n".join(f"* ``np.{x}``" for x in BINARY_ARRAY_TYPES))
 
@@ -135,7 +135,8 @@ def is_timedelta_type(obj: Any) -> TypeGuard[dt.timedelta | np.timedelta64]:
     return isinstance(obj, (dt.timedelta, np.timedelta64))
 
 def convert_date_to_datetime(obj: dt.date) -> float:
-    ''' Convert a date object to a datetime
+    """ Convert a date object to a datetime
+
 
     Args:
         obj (date) : the object to convert
@@ -143,8 +144,17 @@ def convert_date_to_datetime(obj: dt.date) -> float:
     Returns:
         datetime
 
-    '''
-    return (dt.datetime(*obj.timetuple()[:6], tzinfo=dt.timezone.utc) - DT_EPOCH).total_seconds() * 1000
+    """
+    # Fast path if obj is already a datetime *with* tzinfo UTC
+    if isinstance(obj, dt.datetime):
+        if obj.tzinfo is None:
+            obj = obj.replace(tzinfo=dt.timezone.utc)
+        else:
+            obj = obj.astimezone(dt.timezone.utc)
+        return (obj - DT_EPOCH).total_seconds() * 1000
+    # obj is a date (not datetime)
+    obj_dt = dt.datetime(obj.year, obj.month, obj.day, tzinfo=dt.timezone.utc)
+    return (obj_dt - DT_EPOCH).total_seconds() * 1000
 
 def convert_timedelta_type(obj: dt.timedelta | np.timedelta64) -> float:
     ''' Convert any recognized timedelta value to floating point absolute