I think your expression looks good as long as your MerticID and TriageCetegory values are spelled correctly.
Your ArrivalDateTime needs to have a numeric value in order for your modifier to work. If you add the ArrivalDateTime to an list box, are the values left or right aligned as default? If they are left aligned then it's an indication that it's not numeric.
Plees ekskuse my Swenglish and or Norweglish spelling misstakes