Kautilya's Arthashastra 2.20.65

2.20.65

divasasya haraty arkaḥ ṣaṣṭibhāgam ṛtau tataḥ /
karoty ekam ahaśchedaṃ tathaivaikaṃ ca candramāḥ //

The sun carries off (harati) 1/60th of a whole day every day and thus makes one complete day in every two months (ritau). Likewise the moon (falls behind by 1/60th of a whole day every day and falls behind one day in every two months). Thus in the middle of every third year, they (the sun and the moon) make one adhimása, additional month, first in the summer season and second at the end of five years.