Extending Shabbat
Question
A friend invited me to her house on Saturday night 12/21 at 7:00pm and asked me to bring challah for Shabbat. I told her Shabbat was over, but I could still bring the challah. She told me that it's okay to extend Shabbat until Tuesday if you wanted to. I find it hard to believe that you can just change the rules just because you feel like it.
She insists that you can end Shabbat whenever you want up until Tuesday night. I would think you'd need a better reason that just because you feel like it.
What's the Jewish law about choosing your own end time for Shabbat?