Is there a max function for timestamps (with or without timezone)?

When I call max on two timestamps, I get an error :

select max(now()::timestamp, to_timestamp('2021-01-01', 'YYYY-MM-DD')::timestamp);
-- ERROR:  function max(timestamp without time zone, timestamp without time zone) does not exist

My actual usage would be in an upsert query, where I would like to write :

INSERT ...
ON CONFLICT (pk) DO UPDATE SET
  ts = max(table.ts, excluded.ts)

Question

Is there an idiomatic way to take the max between two timestamps in PostgreSQL ?