two different pseudo-code examples to try to better explain what I was trying to achieve.

datetime_update_trigger_1.sql

UPDATE c SET
  CreateDate = CASE WHEN UPDATE ([CreateDateUTC]) THEN dbo.ConvertUTCTimeToPacific(i.CreateDateUTC)
                  ELSE CreateDate
               END,
  CreateDateUTC = CASE WHEN UPDATE ([CreateDate]) THEN dbo.ConvertPacificTimeToUTC(i.CreateDate)
                  ELSE CreateDateUTC
                END
FROM dbo.Calls AS c
JOIN INSERTED AS i
ON i.CallId = c.CallId;

datetime_update_trigger_2.sql

UPDATE c SET
  IF UPDATE ([CreateDateUTC]) -- I'm not sure if this works, but do you need the case statement? (see note 1 below)
    CreateDate = dbo.ConvertUTCTimeToPacific(i.CreateDateUTC)
  IF UPDATE ([CreateDate])
    CreateDateUTC = dbo.ConvertPacificTimeToUTC(i.CreateDate)
FROM dbo.Calls AS c
JOIN INSERTED AS i
ON i.CallId = c.CallId;


/*  Note 1
Can't you conditionally set the value instead of always assigning it and switching whether it overwrites itself or gets converted?
*/

/* Note 2
This does not work
*/