How is the waiting period (iddah) calculated for a widow whose husband died on 1/1/2010? Is it by the Hijri or Gregorian calendar, and when is the last day of her waiting period?

The waiting period (iddah (definition)) for a pregnant woman whose husband has died ends upon her delivery. For a non-pregnant woman, it is four months and ten days according to the lunar calendar, based on the Almighty's saying: "And those of you who die and leave wives behind - they, [the wives], shall wait concerning themselves for four months and ten [days]." If the husband died on the 15th of Muharram 1431 AH, the waiting period ends on the 25th of Jumada al-Ula 1431 AH. If there is doubt about the date of death, it should be considered the 16th of Muharram, and the waiting period would end on the 26th or 27th of Jumada al-Ula, depending on the month. The principle is to be cautious in completing the waiting period with certainty. If the lunar months are unknown, she should observe 130 days as a precautionary measure.