tzais60
This method returns the tzais (nightfall) based on the opinion of the Chavas Yair and Divrei Malkiel that the time to walk the distance of a Mil is 15 minutes for a total of 60 minutes for 4 Mil after sea level sunset. See detailed documentation explaining the 60 minute concept at alos60.
Return
the Date representing 60 minutes after sea level sunset. If the calculation can't be computed such as in the Arctic Circle where there is at least one day a year where the sun does not rise, and one where it does not set, a null will be returned. See detailed explanation on top of the AstronomicalCalendar documentation.