Is it possible for a due date to be off 1month?

Depends : An ultrasound up to 8 weeks gestation is accurate within 3 days. At 12 week, it is accurate within a week. At 20 weeks, it is accurate within 2 weeks. At 28 or more weeks it is accurate within 3 weeks. Your due date by your last menstrual period could be off by a month if it varies from ultrasound dating. If this is true, the ultrasound is more accurate.