Lailatul Qadar/Shab-e-Qadar starts with the sunset and ends at dawn of the next morning.
Keep in mind, when searching for odd nights, do not get confused with the the fasting day. So a 21st night will fall at the breaking of your 20th fast, for example... since Islamic date/day changes at sunset.
Any Islamic reference will be very helpful for that. As people might miss out if they do not pray on the right nights.