formatDate function not working

Hi Forum!

A bit of a head scratcher for me here - I'm trying to use the format date inline function

https://help.nintex.com/en-US/nwc/Content/Designer/Functions/formatDate.htm

I'm doing a build string activity and have tried: 

fn-FormatDate({ItemProperty:EventDate},"MM/DD/YYYY")

and 

fn-FormatDate({ItemProperty:EndDate},"ShortDate")

from the docs, both of those should work.

The outputs I receive are 

09/DD/YYYY

and

S11orPDaPe

What kind of goobledey gook is that?

I'm not sure what I'm doing wrong here.

Best,

Erin