Convert EST to GMT using kibana script field

I have a field of date type(in EST) and I want to convert this to GMT before subtracting from another field using kibana script.
below is the script field definition

doc['re.pdz.end'].value - doc['test.P_START_TIME'].value.getMillis()

I want to convert doc['test.P_START_TIME'] into GMT to get proper millisecond difference since doc['re.pdz.end'] is GMT(UTC) already.

thanks for the help.

I think you'll want to look at the painless API documentation. It looks like there is a class available for an instance of a date that is timezone-aware. You'll want to make a date instance for each field value, and use compareTo to get the time difference. Then add the difference before the subtraction.

Thank you for the way ahead. will check it out.