diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 365cfb1585f00..c78545a12f43b 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -806,6 +806,25 @@ def _fast_union(self, other, sort=None):
         else:
             return left
 
+    def _union(self, other, sort):
+        if not len(other) or self.equals(other) or not len(self):
+            return super()._union(other, sort=sort)
+
+        # We are called by `union`, which is responsible for this validation
+        assert isinstance(other, type(self))
+
+        this, other = self._maybe_utc_convert(other)
+
+        if this._can_fast_union(other):
+            return this._fast_union(other, sort=sort)
+        else:
+            result = Index._union(this, other, sort=sort)
+            if isinstance(result, type(self)):
+                assert result._data.dtype == this.dtype
+                if result.freq is None:
+                    result._set_freq("infer")
+            return result
+
     # --------------------------------------------------------------------
     # Join Methods
     _join_precedence = 10
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 4b2362bea27b3..40d3823c9700b 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -379,25 +379,6 @@ def _formatter_func(self):
     # --------------------------------------------------------------------
     # Set Operation Methods
 
-    def _union(self, other: "DatetimeIndex", sort):
-        if not len(other) or self.equals(other) or not len(self):
-            return super()._union(other, sort=sort)
-
-        # We are called by `union`, which is responsible for this validation
-        assert isinstance(other, DatetimeIndex)
-
-        this, other = self._maybe_utc_convert(other)
-
-        if this._can_fast_union(other):
-            return this._fast_union(other, sort=sort)
-        else:
-            result = Index._union(this, other, sort=sort)
-            if isinstance(result, DatetimeIndex):
-                assert result._data.dtype == this.dtype
-                if result.freq is None:
-                    result._set_freq("infer")
-            return result
-
     def union_many(self, others):
         """
         A bit of a hack to accelerate unioning a collection of indexes.
diff --git a/pandas/core/indexes/timedeltas.py b/pandas/core/indexes/timedeltas.py
index 1bdb4bf8b0a00..ee6e5b984ae7b 100644
--- a/pandas/core/indexes/timedeltas.py
+++ b/pandas/core/indexes/timedeltas.py
@@ -246,24 +246,6 @@ def astype(self, dtype, copy=True):
             return Index(result.astype("i8"), name=self.name)
         return DatetimeIndexOpsMixin.astype(self, dtype, copy=copy)
 
-    def _union(self, other: "TimedeltaIndex", sort):
-        if len(other) == 0 or self.equals(other) or len(self) == 0:
-            return super()._union(other, sort=sort)
-
-        # We are called by `union`, which is responsible for this validation
-        assert isinstance(other, TimedeltaIndex)
-
-        this, other = self, other
-
-        if this._can_fast_union(other):
-            return this._fast_union(other, sort=sort)
-        else:
-            result = Index._union(this, other, sort=sort)
-            if isinstance(result, TimedeltaIndex):
-                if result.freq is None:
-                    result._set_freq("infer")
-            return result
-
     def _maybe_promote(self, other):
         if other.inferred_type == "timedelta":
             other = TimedeltaIndex(other)