The Hijra-Solar (H.S.) calendar

I show a dual-calendar system on many of my blog posts from Kabul, but the second date is not the Islamic Lunar (After-Hijra) calendar. It is the Hijra Solar calendar, used only in Iran and Afghanistan. This calendar is an Islamic calendar because of its start date (622 AD, the year the Prophet left Mecca to govern Medina), but it is not used for religious purposes. As in the rest of the Islamic world, religious days and months are reckoned by the Lunar Hijra calendar, which precesses through the solar year so that months such as Ramadan come more than a week earlier during each solar year.
The Persian solar year always starts on the spring equinox, and is in fact more accurate than either the Julian or Gregorian Christian calendars (see the Wikipedia article for a detailed explanation). Below I have built a correspondence table of Hijra Solar (H.S.) and Gregorian (A.D.) years, beginning with the year in which Sardar Daoud Khan declared H.S. the official state calendar of Afghanistan.

*Because of inaccuracy in the Gregorian calendar, the equinox now occurs on March 20 on every fourth year, beginning in 1996. In those years, the H.S. and A.D. calendars are offset by one day. In the table below I give the start date of Afghan months corresponding to AD years in which the spring equinox begins on March 21; but keep in mind that this must be adjusted for every fourth year until the Christian calendar is corrected.