In the community commons you have a Java action LongToDateTime. You will then get the date you want.
Regards,
Ronald
And no you can not use a microflow to create the date format. You need to create a string attribute and convert the date time to the format you would like in your JSON. See also this info on stackoverflow on the 'right' JSON date format: http://stackoverflow.com/questions/10286204/the-right-json-date-format