Answer:
The way to determine if a particular salat is obligatory on a woman in order to know if she has to do qada of it, she has to look at one or two seconds before the ending time of that salat. By ‘one or two seconds before the ending time of that salat’ we mean such a period of time that she can just say ‘Allah’ of ‘Allahu Akbar’ before the time ends. During this period of time if she is not menstruating then that salat is obligatory upon her. Therefore, if she has not offered it, she has to do qada of it.
On the other hand, if she is menstruating during this time (i.e. one or two seconds before the time ends) then that salat is not obligatory on her. Therefore she does not have to do qada of it. This is regardless of whether she was menstruating or not during the start of the salat time or during the middle of it.
Similarly, if a woman is offering fardh salat and during the salat she starts menstruating, she will not do qada of it. However, if she starts menstruating while offering nafl salat, she has to do qada of it.
Allaamah Ibn Aabideen states in his commentary of Allaamah Birgivi’s treatise on the ahkaam of menstruation:
“In order to determine that the salat is prohibited and not obligatory (on a woman), the end time of every salat – the period of time in which one can say Allah — is to be taken into consideration… If she is menstruating at this time, then it is not obligatory on her to offer that salat, neither as adaa nor as qadaa.”
(Rasaail Ibn Aabideen vol. 1 pg. 110)
In Al-Fataawa Al-Hindiyya it is stated:
“If a woman begins the (fardh) salat towards the end of its time and then she menstruates, she does not have to do qadaa of this salat, as opposed to offering nafl salat (i.e. in which case she will have to do qadaa of it)”
(Al-Fataawa Al-Hindiyya vol. 1 pg. 42)
Mufti Faizal Riza