import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.event.type.EventDispatchOption

def cfm = ComponentAccessor.customFieldManager
def issueManager = ComponentAccessor.issueManager

def startDate = cfm.getCustomFieldObjectByName("Downtime From")
def endDate = cfm.getCustomFieldObjectByName("Downtime To")
def days = cfm.getCustomFieldObjectByName("Downtime Durration")

def issueStartDate = issue.getCustomFieldValue(startDate) as Date
def issueEndDate = issue.getCustomFieldValue(endDate) as Date

def duration
use(groovy.time.TimeCategory) {
    duration = ((issueEndDate.getTime() - issueStartDate.getTime()) / 1000) / 60
}

issue.setCustomFieldValue(days,duration.toString())
issueManager.updateIssue(ComponentAccessor.jiraAuthenticationContext.loggedInUser, issue, EventDispatchOption.DO_NOT_DISPATCH, false)