Calculate the Difference Between Two Dates

import com.atlassian.jira.component.ComponentAccessor

import java.sql.Timestamp
import java.time.temporal.ChronoUnit

// Get the required component
def customFieldManager = ComponentAccessor.customFieldManager

// The name of the lower date custom field
final String lowerDateCustomFieldName = "Lower Date Custom Field"
// The name of the higher date custom field
final String higherDateCustomFieldName = "Higher Date Custom Field"

// Get the custom field objects
def lowerDateCustomField = customFieldManager.getCustomFieldObjects(issue).find { it.name == lowerDateCustomFieldName }
def higherDateCustomField = customFieldManager.getCustomFieldObjects(issue).find { it.name == higherDateCustomFieldName }
if (!lowerDateCustomField || !higherDateCustomField) {
    log.info "Could not find one ore more of the provided custom fields"
    return null
}

// Get the date values from both issues
def lowerDateValue = issue.getCustomFieldValue(lowerDateCustomField) as Timestamp
def higherDateValue = issue.getCustomFieldValue(higherDateCustomField) as Timestamp
// Transform both values to instants
def lowerDateInstant = lowerDateValue?.toInstant()
def higherDateInstant = higherDateValue?.toInstant()

// Change the chrono unit to obtain the difference in other time unit.
final chronoUnit = ChronoUnit.DAYS
// Calculate the difference between the lower and the higher date.
lowerDateInstant && higherDateInstant ? chronoUnit.between(lowerDateInstant, higherDateInstant) : null