It's based on when you started and finished the level, not session age.
How is this evaluating your efficiency? And how is this a fair approach for everyone? I'd wished it was session-age. RL time favors so much more than just your efficiency: your sleeping times / how far you pushed idle time, your Auto-advs (despite their own inefficiency), your use of powers. And some of those points even beat efficiency in how much they effect the level times. Someone who is only idling at times will never beat those limits if they are compared to RL time, they could beat session-age-limits, though.