PostFunction to set Date-Time custom field with Groovy Script Runner

import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.customfields.CustomFieldType
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.ModifiedValue
import com.atlassian.jira.issue.util.DefaultIssueChangeHolder
import com.atlassian.jira.issue.util.IssueChangeHolder

ComponentManager componentManager = ComponentManager.getInstance()
CustomFieldManager customFieldManager = componentManager.getCustomFieldManager()
CustomField cf = customFieldManager.getCustomFieldObject("customfield_18204")
val = issue.getCustomFieldValue(cf)
if ( val == null) {
IssueChangeHolder changeHolder = new DefaultIssueChangeHolder();
t0 = new java.sql.Timestamp(System.currentTimeMillis())
cf.updateValue(null, issue, new ModifiedValue(issue.getCustomFieldValue(cf),t0), changeHolder)
}