An english translation of the SURYA-SIDDHANTA has been uploaded on this site library under the REFERENCES index, INDIAN TRADITIONS header : http://prajnaquest.fr/downloads/BookofDzyan/IndianTraditions/Others/Surya%20Siddhanta.pdf
It was translated from the sanskrit by Pandit Bâpû Deva Shâstri, and published in 1860 by the Asiatic Society of Bengal.
It is bounded with a translation of the SIDDHANTA SHIROMANI, translated by Lancelot Wilkinson and Pandit Bâpû Deva Shâstri, published in 1861.
In his postscript of the SURYA-SIDDHANTA, Bâpû Deva explained that eighteen Siddhanta were written, of which only four were procurable during his time (SÛRYA-Sid., BRAHMA-Sid., SOMA Sid., and VASISHTA-Sid.). The SURYA Sid. is supposed to be the oldest.
Page 108 (SIDDHANTA SHIROMANI) gives another calculation for the elapsed-time-since-the-beginning-of-this-kalpa :
“Of the present KALPA, 6 Manus with their 7 Sandhis, 27 Yugas ans their 3 Yuga’Nghri (Krita, Treta, and Dwapara), and 3179 sidereal years of the fourth Yuga’Nghri of the 28th Yuga of the 7th Manu, that is to say 1,972,947,179 sidereal years have elapsed from the beginning of the present Kalpa to the commencement of the Sa’liwa’hana era.”
Note : Saliwahana was a king of southern India, born in 78 A.D., starting the “Saka Era”.
Compared to H.P.B. number given in the S.D (vol II, page 68), there is a gross difference of 1,972,947,179 – 1,955,884,687 = 17,062,492 years for the elapsed time since the beginning of cosmic evolution.
The difference of about 17 million years in the number given in the Siddhanta Shiromani is because this treatise did not deduct the time taken for “creation” given in the Surya Siddhanta. The Surya Siddhanta says that we must allow the time for “creation” (sṛṣṭi, emanation or manifestation) at the beginning of the kalpa, 17,064,000 years, before commencing the calculations of the elapsed time since the beginning of cosmic evolution. The Siddhanta Shiromani, like some other Sanskrit texts, did not do this. HPB followed the Surya Siddhanta reckoning. It is good to have both the Surya Siddhanta and the Siddhanta Shiromani available here for comparison.