Solved
Parse/Format DateTime linked to wrong documentation
Parse/Format DateTime help is linked to https://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html for documentation on the formatting codes to use. However, using e.g., u gets an IllegalArgumentException. From the exception it appears Tasker is using Joda, so the doc link should be https://www.joda.org/joda-time/apidocs/org/joda/time/format/DateTimeFormat.html instead.