Custom/formatted handlebars - IoT Gateway Advanced Template

I know the idea of using REST Client is to push data that makes sense and let the REST Server handle the other side of things.

Howerver, the "otherside" in my case isn't as flexible. And the usecase am in, things will work only if the "|SERVERDATE|" send the actual date in yyyy-mm-dd format

Anywhere around having other handlebars variables to be used outside the |VALUES| part of the advanced template?