Surya Siddhanta
युगस्य दशमो भागश् चतुस्त्रिद्व्येकसङ्गुणः। क्रमात् कृतयुगादीनाम् षष्ठाम्शः सन्ध्ययोः स्वकः ॥
The tenth part of 4,320,000 the number of years in a great yuga, multiplied by 4, 3, 2, 1 respectively make up the years of each of the four yugas, Krịta and others, the years of each yuga include their own sixth part, which is collectively the number of years of Sandhyá and Sandhyáns'a, (the periods at the commencement and expiration of each yuga).
english translation
yugasya dazamo bhAgaz catustridvyekasaGguNaH। kramAt kRtayugAdInAm SaSThAmzaH sandhyayoH svakaH ॥
hk transliteration by Sanscript