Witr prayers can be offered any time after Isha prayers up until Fajr time. The Holy Prophet^(sa) used to offer Witr prayers at the end of Tahajjud. However, they can be offered any time between Isha and before Fajr time.

Hazrat Umar^(ra) would offer Witr prayers after Tahajjud whereas Hazrat Abu Bakr^(ra) would offer them after Isha.

The Promised Messiah^(as), Hazrat Mirza Ghulam Ahmad would offer his Witr prayers after Isha before going to sleep, and later, after awakening, would then offer Tahajjud prayers in the morning.

In Sunan Abi Dawud it is narrated:

“The Holy Prophet^(sa) asked Abu Bakr^(ra): ‘When do you observe the Witr?’

“He replied: ‘I observe the Witr prayer in the early hours of the night.’

“The Holy Prophet^(sa) asked Umar^(ra): ‘When do you observe the Witr?’

“He replied: ‘At the end of the night.’

“He then said to Abu Bakr^(ra): ‘This one has followed it with care’; and he said to Umar^(ra): ‘He has followed it with strength.’” (Sunan Abi Dawud, The Book of prayer, Detailed Injunctions about Witr)

The Witr prayers should not be missed and if one is doubtful as to if they will get up and offer them time after sleeping, then it is better to offer them before sleeping after Isha prayers.