[template][time] Add template time platform (#14838)

Co-authored-by: clydebarrow <2366188+clydebarrow@users.noreply.github.com>

tests/integration/test_template_time.py

@@ -0,0 +1,49 @@
+"""Integration test for the template time platform."""
+
+from __future__ import annotations
+
+import asyncio
+
+from aioesphomeapi import EntityState, TextSensorState
+import pytest
+
+from .state_utils import InitialStateHelper, build_key_to_entity_mapping
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+# Timestamps are exposed as text sensors (rather than 32-bit float sensor states,
+# which cannot represent a UNIX epoch exactly) so the exact value can be checked.
+EXPECTED_STATES = {
+    "fixed_timestamp": "1700000000",
+    "offset_timestamp": "1700003600",
+    "empty_time_valid": "invalid",
+}
+
+
+@pytest.mark.asyncio
+async def test_template_time(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Verify the template time platform evaluates its lambda on demand."""
+    async with run_compiled(yaml_config), api_client_connected() as client:
+        entities, _ = await client.list_entities_services()
+
+        key_to_name = build_key_to_entity_mapping(entities, list(EXPECTED_STATES))
+        events = {name: asyncio.Event() for name in EXPECTED_STATES}
+
+        def on_state(state: EntityState) -> None:
+            if isinstance(state, TextSensorState) and not state.missing_state:
+                name = key_to_name.get(state.key)
+                if name is not None and state.state == EXPECTED_STATES[name]:
+                    events[name].set()
+
+        initial_state_helper = InitialStateHelper(entities)
+        client.subscribe_states(initial_state_helper.on_state_wrapper(on_state))
+        await initial_state_helper.wait_for_initial_states()
+
+        for name, event in events.items():
+            try:
+                await asyncio.wait_for(event.wait(), timeout=3.0)
+            except TimeoutError:
+                pytest.fail(f"Timeout waiting for {name} to report its expected value")