Editing Iranian calendars (section)

=== Modifications by Parthians, Ardashir I, Hormizd I, Yazdgerd III ===
The [[Parthia]]ns (Arsacid dynasty) adopted the same calendar system with minor modifications, and dated their era from 248 BC, the date they succeeded the Seleucids. Their names for the months and days are Parthian equivalents of the Avestan ones used previously, differing slightly from the [[Middle Persian]] names used by the [[Sassanians]]. For example, in Achaemenid times the modern Persian month 'Day' was called Dadvah (Creator), in Parthian it was Datush and the Sassanians named it Dadv/Dai (Dadar in Pahlavi).

When in April of AD 224 the Parthian dynasty fell and was replaced by the Sasanid, the new king, [[Ardashir I]], abolished the official Babylonian calendar and replaced it with the Zoroastrian.   This involved a correction to the places of the ''gahanbar'', which had slipped back in the seasons since they were fixed.   These were placed eight months later, as were the ''epagomenai'', the 'Gatha' or 'Gah' days after the ancient Zoroastrian hymns of the same name.   Other countries, such as the Armenians and Choresmians, did not accept the change.   The new dates were:

{|class="wikitable" style="float:centre"
!No.||Name||Achaemenid||Choresmian||Sasanian||Time since previous
|-
|1||maidyozarem||(11-) 15 ii (Ardawahisht)||15 v||(11-) 15 x (Day)||45 days
|-
|2||maidyoshahem||(11-) 15 iv (Tir)||15 vii||(11-) 15 xii (Spandarmad)||60 days
|-
|3||paitishahem||(26-) 30 vi (Shahrivar)||30 ix||(26-) 30 ii (Ardawahisht)||75 days
|-
|4||ayathrem||(26-) 30 vii (Mihr)||30 x||(26-) 30 iii (Khordad)||30 days
|-
|5||maidyarem||(11-) 15 x (Day)||10 i||(11-) 15 vi (Shahrewar)||75 days
|-
|6||hamaspathmaidyem||(1-) 5 Epagomene||30 iii||(1-) 5 Epagomene||80 days
|}

In AD 224 the vernal equinox at Greenwich fell at noon on 21 March, which was 22 Shahrewar.   Immediately after the reform 21 March corresponded to 27 Shahrewar.   Here is the calendar for AD 225–6:

{|class="wikitable" style="float:centre"
|+* = 1 Epagomene
!Armenian<br /> month||First day||Egyptian<br /> month||First day||Persian<br /> month||First day
|-
|1||26* September–1 October||4||26 September||1||26 September
|-
|2||31 October||5||26 October||2||26 October
|-
|3||30 November||6||25 November||3||25 November
|-
|4||30 December||7||25 December||4||25 December
|-
|5||29 January||8||24 January||5||24 January
|-
|6||28 February||9||23 February||6||23 February
|-°
|7||30 March||10||25 March||7||25 March
|-
|8||29 April||11||24 April||8||24 April
|-
|9||29 May||12||24 May||9||24*–29 May
|-
|10||28 June||1||23*–28 June||10||28 June
|-
|11||28 July||2||28 July||11||28 July
|-
|12||27 August||3||27 August||12||27 August
|}

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''.