The waajib prayers – which are not farD – which have a specified time are the witr prayer and the two Eid prayers according to Abu Hanifah.
As for the witr prayer, Abu Hanifah holds the opinion that its time begins at the same time when Ishaa begins which is when the white shafaQ disappears. However, he says that one must maintain the order of the two prayers. One prays Ishaa. Then, one prays the witr prayer.
The majority of the scholars, including the two students of Abu Hanifah, hold the opinion that the time for the witr prayer begins after one has prayed the Ishaa prayer.
The difference between the majority and Abu Hanifah has a practical outcome.
Take, for example, a person who prays Ishaa without Wuduu, forgetfully. Then, he does Wuduu and prays the witr prayer. After that, he remembers that he prayed Ishaa without Wuduu.
According to Abu Hanifah, the person would repeat Ishaa but does not have to repeat the witr prayer because he prayed Ishaa without Wuddu but as for the witr prayer he prayed it with wuduu in its time so there is not need to repeat it.
As for the majority scholars, the person will repeat Ishaa because he prayed it without Wuduu, and he will also repeat the witr prayer because though he prayed the witr prayer with wuduu he prayed it when it was not time for it (remember the majority hold that the time of the witr begins after one has prayed Ishaa).
Kuwuaiti Encyclopedia of fiqh
Volume 7 Page 179