Discussed this with @jdconrad

Date formatting and manipulation in Painless currently relies on using native Java methods which are probably not performant. Example - suppose i want to show dates by day (mon-sun). This is a reasonably common usecase. I resort to:

new SimpleDateFormat('E').format(new Date(doc['created_on'].value))

Proposal is for a simple set of date parsing functions - to and from string.