Maharishi Lagadha

Rishi Lagadha benefited Bharatvarsha by his knowledge about three 3000 years ago.
He composed a Granth called Vedang Jyotish.
It is probably the oldest scripture of Mathematics.
The was the Lagadha Rishi who created systematic format for Date calculation.

He Presumed One Yuga has five Varshas (saṃvatsara, parivatsaraḥ, idāvatsara, idvatsara & Vatsar) and divided Each year into 2 Aynas, 6 ṛtuḥs & 12 months.

2 Aynas are uttarāyana & dakṣiṇāyana and ṛtuḥs are śiśiraḥ, vasantaḥ, grīṣmaḥ, varṣā, śarad & . hemantaḥ & 12 months are as followed tapaḥ , tapasyaḥ, madhuḥ, mādhava, śukra, śuci, nabhaḥ, nabhasyaḥ, iṣa, ūrjaḥ, saha & sahasya.

which are also called māgha , phālgunaḥ caitraḥ, vaiśākhaḥ, jyeṣṭha, āṣāḍhaḥ, śrāvaṇaḥ, bhādra, aśvin, kārtikaḥ, mārgaśīrṣa & Posh. Every month has 2 sides in each month śuklapakṣa and kṛṣṇapakṣa.

There are 15-15 days in both पक्ष. And 15 muhūrta in the day and 15 muhūrta in the night were decided. 30 day months firstly introduced by him. He also introduced concept of adhimāsaḥ to make coordination between Solar years and Lunar years,

according to this in every three years there is one year which has 13 months.

Thanks