8 week ultrasound showed fetal pole that measure 6.2 mm with no heartbeat. is it possible my dates are off even if i am sure of my last period?

It's possible: Just because you know when your last period happened, doesn't mean you know your exact date of conception. Unfortunately, seeing a fetal pole of that size without a heartbeat is still cause for concern. We usually see cardiac activity at around 5 mm, but that is not written in stone. It is best to repeat the ultrasound in a week or so, and if there is still no heartbeat, then miscarriage is likely