Some airlines allow you to book one-way awards. So you can book your departure at a later time.
Of course, you may have to sacrifice the option to have open jaw flights or stop overs using this method. Those are usually only available on round trip bookings.
Also, once you have booked the one way outbound, and the one-way return, you can call the airline and see if they will allow you to combine into a round trip PNR, but this may have fees associated, as it is technically redepositing the miles into your account and issuing a new award. But it may be worth it to secure the flights that you want.
Here are some United threads that may help:
Saver-award-inventory-not-released-far-out-merged
Open-jaw-stopover-award-flights-consolidated