I would really suggest against that if you have more than just a few records of data. You should reindex your data and make sure date time is stored correctly. Doing this with scripted fields will be painfully slow. Not 100% but i guess elastic will always have to go over every record and do conversion and then return the subset you are interested in.
that being said, if you look at painless reference: https://www.elastic.co/guide/en/elasticsearch/painless/master/painless-api-reference.html
you can see it supports SimpleDateFormat
looking at oracle documentation: https://docs.oracle.com/javase/6/docs/api/java/text/SimpleDateFormat.html
something like this should work:
new SimpleDateFormat('dd/MMM/yyyy:HH:mm:ss Z').parse(doc['fieldname'].value);