import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.IssueManager
import java.sql.Timestamp
def issueManager = ComponentAccessor.getIssueManager()
MutableIssue myIssue = issueManager.getIssueObject("TESTTEST-1")
Date newdate = new Date("12/12/2019");
myIssue.setCreated(newdate.toTimestamp())
myIssue.store()