Okay, here's why we shouldn't do that. I'll use player 1 and 2 as examples. Let's say player one if from London, and we decide the riddle should be posted noon London time. Player 2 can be from Orlando, with a time of -5. Let's say player 1 sends in the answer to their riddle at 1PM their time, because they logged on just as the riddle was posted. Now, player two logs on at 11AM their time (Making it 4PM in London), solves the riddle, and waits until noon exactly to send it in. This makes the time they took to solve it seem like 0:00 even if they took x amount of time.
Woon should just post the riddle, whenever, and award people in whatever order the PMs came in.