Calculate the Difference Between Two Dates

import java.time.ZonedDateTime
import java.time.format.DateTimeFormatter
import java.time.temporal.ChronoUnit

// The issue key
final issueKey = 'TEST-1'
final dateFieldName = 'Created'
final chronoUnit = ChronoUnit.DAYS

// Jira datetime field format
def formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSSZ")

// Pick a date that you would like to calculate from
def issueFieldId = getFieldIdFromName(dateFieldName)
// Pick a date from another date field, in this case the built-in 'Updated' field
def updatedFieldId = 'updated'

def createdDate = ZonedDateTime.parse(getIssueField(issueKey, issueFieldId), formatter)
def updatedDate = ZonedDateTime.parse(getIssueField(issueKey, updatedFieldId), formatter)

def dateDifference = chronoUnit.between(createdDate, updatedDate)

/**
 * Get the issue field data on a given issue
 * @param issueKey The key of the issue to get the data from
 * @param fieldId The field to get the data of
 * @return The value of the field
 */
String getIssueField(issueKey, fieldId) {
    def issueFieldValue = null
    def result = get('/rest/api/2/issue/' + issueKey)
        .header('Content-Type', 'application/json')
        .asObject(Map)
    if (result.status == 200) {
        result.body.fields.each { key, value ->
            if (key == fieldId) {
                issueFieldValue = value.toString()
            }
        }
        logger.warn "${issueFieldValue}"
        return issueFieldValue
    }

    logger.warn "Failed to find issue: Status: ${result.status} ${result.body}"
    null
}

/**
 * Get the id of a field
 * @param fieldName The name of the field
 * @return The field id
 */
String getFieldIdFromName(String fieldName) {
    def fields = get("/rest/api/2/field").asObject(List).body
    def customFieldObject = (fields as List<Map>).find { Map field ->
        field.name == fieldName
    }
    (customFieldObject as Map).id
}

// Return the value
"${dateDifference} ${chronoUnit.toString().toLowerCase()}"