Need help with Sportline chest strap

Hi everyone, I take a gym class a few times a week and went out last night and bought a Sportline chest strap and watch to monitor HR and Calories burned. Here is my question.... I get to gym and change and put on strap and watch, class is 30 minutes, the I go straight in to change and jump in car and rush back to work to my desk job. Do I (A) remove the chest strap right after class and use those calories like today said 265 or (B) wait til I get back to my desk when my heart rate has gone where right now it says 310 calories burned.

What calories do I log?