tzais90Zmanis
Method to return tzais (dusk) calculated using 90 minutes zmaniyos or 1/8th of the day after seaLevelSunset. This time is known in Yiddish as the achtel (an eighth) zman.
Return
the Date representing the time. 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.