Sort a lookup checklist by date

Sort a lookup checklist by date

Is it possible to sort a lookup checklist by date? When I click the box for "Show choices in alphabetical order" it puts the dates in the wrong order because the day is listed first in this format dd - Month - yyyy. I adjusted the date format under application settings and this does not fix how the date displays in the lookup checklist.

I considered using a date function to lookup the month, then the date, and combine those two together by a formula so that it would appear correctly when alphabetized, but I get an error message when I insert that into my formula:

Variable 'Formula_11' is not declared.

I created Formula_11 to contain the number of the month like so:

Event_Date.getMonth()

Any suggestions?

Thanks.