Add easy converting string timestamps/dates to datetime objects in templates (#51576)

tests/helpers/test_template.py

@@ -526,6 +526,33 @@ def test_timestamp_local(hass):
         )
 
 
+@pytest.mark.parametrize(
+    "input",
+    (
+        "2021-06-03 13:00:00.000000+00:00",
+        "1986-07-09T12:00:00Z",
+        "2016-10-19 15:22:05.588122+0100",
+        "2016-10-19",
+        "2021-01-01 00:00:01",
+        "invalid",
+    ),
+)
+def test_as_datetime(hass, input):
+    """Test converting a timestamp string to a date object."""
+    expected = dt_util.parse_datetime(input)
+    if expected is not None:
+        expected = str(expected)
+
+    assert (
+        template.Template(f"{{{{ as_datetime('{input}') }}}}", hass).async_render()
+        == expected
+    )
+    assert (
+        template.Template(f"{{{{ '{input}' | as_datetime }}}}", hass).async_render()
+        == expected
+    )
+
+
 def test_as_local(hass):
     """Test converting time to local."""