create an new custom date field
use one of the script addons of the Marketplace to calculate the difference
https://marketplace.atlassian.com/apps/27136/jira-misc-custom-fields?hosting=datacenter&tab=overview
calculate one or use the "Calculated Time in status custom field"
or
https://marketplace.atlassian.com/apps/1215460/automation-for-jira?hosting=cloud&tab=overview
and many other addons
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