How to create new calculation field?

I am using Prometheus to get data to my metric.
I have 2 metric from Prometheus


my_metric {key="01234",endpoint="http-metrics",instance="",job="my-metrics",time1="2020-08-09 14:04:21 +0000 UTC",time2="2020-08-10 12:12:26 +0000 UTC",type="error}

This is the query I did
( ((first_metric{status=“error”} == 1 ) + on(key) group_left(time,time2) my_metric{type=“error”})

I want to create new calculation field in grafana that will do time1 - time2
is it possible ?
How I can do it ?