I bought one from travel agent; I did not get a chance to choose the time. I have to fly to Washington in the morning and wait for 8 hours. Another 6 hours in Doha.
But if you buy online, you will see different choices with similar price of a little more. You can choose the one that fits to you schedule and has less awaiting period on different airport.
Same applies while returning back. Buying online is what you see and pay for it as per you choice.