This allows auto due date of 30 days, but i want to also add, but if custom field = text then auto due date of 15 days.
import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.customfields.CustomFieldType
import com.atlassian.jira.issue.fields.CustomField
import java.sql.Timestamp;
MutableIssue myIssue = issue
Calendar cal = Calendar.getInstance();
// set due date to: current date + 30 days
Timestamp mydueDate = new Timestamp(cal.getTimeInMillis()+ 2592000000);
myIssue.setDueDate(mydueDate);