On the Indian solar year
This work is the third appendix in the book Historia regni Graecorum Bactriani (History of the Bactrian kingdom of the Greeks), written by Euler's friend and St. Petersbrg Academy colleague T. S. Bayer. The original appendices were written by a Danish missionary in Tranquebar, C. T. Walther ("The Indian Doctrine of Time," pp. 145-190), and by Bayer himself, based on his correspondence with Walther and other Tranquebar missionaries ("Supplement to the Indian Doctrine of Time," pp. 191-200). Euler's contribution appears immediately after these. (Based on Kim Plofker's translation introduction.)
Original Source Citation
Historia Regni Graecorum Bactriani, by T. S. Bayer, Volume , pp. 201-213.
Opera Omnia Citation
Series 2, Volume 30, pp.5-12.