Surya Siddhanta
सन्ध्यासन्ध्याम्शसहितम् विज्ञेयम् तच्चतुर्युगम्। ख कृतादीनाम् व्यवस्थेयम् धर्मपादव्यवस्थया ॥
These four yugas including their Sandhyá[2] and Sandhyáns'a contain 4,320,000 solar years. The numbers of years included in these four small yugas are proportional to the numbers of the legs of Dharma[3] (virtue personified).
english translation
sandhyAsandhyAmzasahitam vijJeyam taccaturyugam। kha kRtAdInAm vyavastheyam dharmapAdavyavasthayA ॥
hk transliteration by Sanscript