In 538 BC Cyrus the Great (uncertain if he was a Zoroastrian) conquered Babylon and the Babylonian luni-solar calendar came into use for civil purposes. He was accompanied by Darius, a Zoroastrian who became ruler of the Persian empire in 517 BC.
The Zoroastrians adopted the wandering Egyptian solar calendar of twelve months of thirty days plus five epagomenal days.
The Gregorian calendar is the calendar that is used nearly everywhere in the world.
Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the solar observation directly and modified for their beliefs. The months had two or three divisions depending on the phase of the moon.
A short table of year correspondences between the Persian and Gregorian calendars is provided below.
Although the earliest evidence of Iranian calendrical traditions is from the second millennium BCE, predating the appearance of the Iranian prophet Zoroaster, the first fully preserved calendar is that of the Achaemenids.
As their year began in the spring (with the festival of nowruz) the epagemonai were placed just before nowruz.
In Egypt the star Sirius had significance since every 1460 years (the Sothic cycle) its heliacal rising (just before sunrise) marked the Egyptian new year and the inundation of the Nile.