Rule of thumb when scheduling interviews (face-to-face or over the phone): try to avoid inconvenient time like early morning, aroung lunch time, and near end of work day.
If your scheduled interview at 12 and HR called you at 11, just tell her/him sorry I thought our interview was at 12. I am on my way back home so can I call you around 12? You have the right to stick to the original schedule because it was agreed by both of you. It was unprofessional of her to change it without obtaining you consent first, or at least she should offer some good reasons/explanations for that.