What are the rulings regarding prayer and purification concerning irregular bleeding, and is it permissible for a woman experiencing irregular bleeding (mustahaḍah) to combine prayers with ghusl or ablution, and what is the ruling on blood remaining inside the vagina, and can the Gregorian calendar be used to calculate menstruation?

The default rule for menstruation is that it continues until the sighting of purity. If it exceeds 15 days, it becomes Istihada (non-menstrual bleeding). In such a case, the woman reverts to her known regular habit before the disruption. If she does not have a habit, she distinguishes the blood (between menstrual and non-menstrual). Otherwise, she follows the common habit of her female relatives. A woman experiencing Istihada performs ablution for each prayer after its time has entered. It is permissible for her to combine two prayers if performing ablution for each prayer becomes difficult for her. The menstrual cycle should be calculated according to the Hijri calendar, as it is the one considered valid in Islamic law for determining timings and rulings.