The nirayana year is the sidereal year, that is, is the actual time required for the Earth to revolve once around the Sun with respect to a fixed point on the ecliptic, and its duration is approximately 365.256363 days (365 days 6 hours 9 minutes 10 seconds).
Since the stars are fixed with respect to the ecliptic, the starting point remains unchanged, hence the name nirayana.
[3][4] vaiśākha jyaiṣṭha āṣāḍha śrāvaṇa bhādrapada āśvina kārttika mārgaśīrṣa pauṣa māgha phalguna caitra In the calendars that follow the nirayana system, a month is an artificial unit of time.
The exact moment at which a new nirayana month begins can occur at any time of day, early morning, evening or night.
This is because the seasons depend on the position of the sun on the ecliptic relative to the celestial equator.
But in the nirayana system, this happens not on the 22nd December but on the 14th January and the winter season is also supposed to begin on that date.