I'm going to leave in the middle of a date to give my dad a ride home
Do you really need to do that? Why would you leave in the middle of a date?
My father needs a ride home before 11 PM
If you cannot keep the date with your girlfriend than you should schedule it for another night. It is not appropriate to walk out on a date and your girlfriend could interpret your behavior as uncaring and rude.
I can walk out if I like. My dad is more important than my girlfriend.
It's wonderful that you care deeply about your dad, but don't you think your girlfriend matters a lot too? How would she feel being stood up? How about trying to respect her time next date, so you can fully enjoy things together.