Function getYahrzeit

Calculates yahrzeit. hyear must be after original date of death. Returns undefined when requested year preceeds or is same as original year.

Hebcal uses the algorithm defined in "Calendrical Calculations" by Edward M. Reingold and Nachum Dershowitz.

The customary anniversary date of a death is more complicated and depends also on the character of the year in which the first anniversary occurs. There are several cases:

• If the date of death is Marcheshvan 30, the anniversary in general depends on the first anniversary; if that first anniversary was not Marcheshvan 30, use the day before Kislev 1.
• If the date of death is Kislev 30, the anniversary in general again depends on the first anniversary — if that was not Kislev 30, use the day before Tevet 1.
• If the date of death is Adar II, the anniversary is the same day in the last month of the Hebrew year (Adar or Adar II).
• If the date of death is Adar I 30, the anniversary in a Hebrew year that is not a leap year (in which Adar only has 29 days) is the last day in Shevat.
• In all other cases, use the normal (that is, same month number) anniversary of the date of death. [Calendrical Calculations p. 113]

import {getYahrzeit} from '@hebcal/hdate';
const dt = new Date(2014, 2, 2); // '2014-03-02' == '30 Adar I 5774'
const anniversary = getYahrzeit(5780, dt); // '2/25/2020' == '30 Sh\'vat 5780'
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