text: The change caused confusion and was immensely unpopular.   The new epagomenai were referred to as "robber days".   The people now observed the "Great" nowruz on 6 Frawardin, which was Zoroaster's birthday and corresponded to 1 Frawardin in the old calendar.   The new 1 Frawardin was observed as the "lesser" nowruz.   Hormizd I (AD 272–273) made the intervening days into festivals as well.   In AD 273, the vernal equinox at 0° fell at 05:00 UTC on 21 March.
  Yazdegerd I reigned from AD 399–420.   In AD 400 the equinox fell about 19 March, which was 9 Aban.   According to al-Biruni, in that reign there was a double adjustment of the start of the araji year.   The tenth-century astronomer Abu'l-asan Kusyar noted that during the reign of Osrow II (AD 589–628) the sun entered Aries in Adur.   This happened throughout his reign.   An araji era was introduced dating from AD 621, and the Yazdegerdi era dates from 16 June AD 632, so the Yazdegerdi era is eleven years behind the araji.
  Muslim conquest
  The Muslim rulers who took over from the middle of the seventh century used the Islamic calendar for administration, which caused hardship because the year was shorter – i.e. a tax which was formerly collected after the harvest would now have to be paid before the harvest.   Traditionally it is said that the caliph Omar reintroduced the Persian calendar for tax collection purposes.