Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v1.c5.26

v1.c5.26

As we are these days more familiar with the Christian era than the Śaka era, it is more convenient to discuss the dates of events in terms of Christian era. Our astronomical works, however, invariably use the Śaka era. Let the work belong to any part of India whatsoever, it is bound to use the Śaka year, even if it is not in use in every day life there. I have accordingly used the Śaka era almost everywhere in the book. However, the year indicated as B. S. (before Śaka) can safely pass for B. C., because the difference of 78 years is negligible where dates of very remote antiquity are concerned. Everywhere in this book the Śaka figure indicates the expired year unless it is specifically stated to be current. The planetary positions should be taken as Nirayaṇa or as calculated by the Graha-lāghava system unless the word Sāyaṇa is specifically used. Words like Sūrya-Siddhānta, Ārya-Siddhānta, and Brahma-Siddhānta used without qualification, should be taken to mean the current or later Sūrya-Siddhānta, the first Ārya-Siddhānta and the Brahmagupta-Siddhānta respectively.