I tried your suggestion, but this it’s not working for me:
type: "custom:secondaryinfo-entity-row"
secondary_info: "Last Update: [[ {{ as_timestamp(states.sensor.MYSENSORID.attributes.time) | timestamp_custom('%D %-I:%M %P') }} ]]"
However
{{ as_timestamp(states.sensor.MYSENSORID.attributes.time) | timestamp_custom('%D %-I:%M %P') }}
is working well in the template page under developer tools.
I reed here that secondary info don’t support jinja. So I think that’s the problem.