Computing time interval between 2 timestamp string fields in Elasticsearch

Hi, I am a newbie to grafana, and was wondering if it can be done (and any pointers/docs on how to achieve it would be greatly appreciated): I have two timestamp fields in Elasticsearch (stored as string - not as long in milliseconds), is there a way for grafana to retrieve the 2 fields and compute the time interval difference between the 2 fields and display the difference? Thanks in advance.