Your date is not actually a Julian Date (which is defined as the number of days that passed since a certain start day in the past). However, getting your result is very easy ;):
OK, in words: first of all, take today’s year and subtract 2000 (in order to get to “18”, different windowing would also be possible of course) and format it as a string. The second part is harder - it uses a DateDiff in order to get the number of days between today and Jan. 1st this year (i.e. it returns the day of year). As you wanted to get “333”, it’s adding one more day. In order to get rid of fractions, it uses an Int() function and then - as you want to concatenate it to the “18”, converts it to a string as well. Thanks for this very interesting challenge!