timeSinceChangedMs doesn’t exist in this context only the text timestamps from Home Assistant.
You can use a JSONata expression to convert them to a timestamp and check that way.
$millis() - $toMillis($entity().last_changed) > 3600000
[{"id":"da45ecf3.23b07","type":"ha-get-entities","z":"ffbd7f06.4a014","name":"","rules":[{"property":"entity_id","logic":"starts_with","value":"sensor.room","valueType":"str"},{"property":"ANY TEXT NEEDS TO BE HERE","logic":"jsonata","value":"$millis() - $toMillis($entity().last_changed) > 3600000","valueType":"jsonata"}],"output_type":"array","output_empty_results":false,"output_location_type":"msg","output_location":"payload","output_results_count":1,"x":320,"y":1248,"wires":[["4a966b2b.ce1dd4"]]},{"id":"225bd07a.1d69b","type":"inject","z":"ffbd7f06.4a014","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":140,"y":1248,"wires":[["da45ecf3.23b07"]]},{"id":"4a966b2b.ce1dd4","type":"debug","z":"ffbd7f06.4a014","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":550,"y":1248,"wires":[]}]
Found a bug, while making this example, that requires the property field not to be empty but the example should still work.
edit: I’ll see about adding in timeSinceChangedMs as a filterable property