sandhya-sandhyamsayor antar
yah kalah sata-saṅkhyayoh
sandhya-transitional period before; sandhya-amsayoh-and transitional period after; antah-within; yah-that which; kalah-duration of time; sata-saṅkhyayoh-hundreds of years; tam eva-that period; ahuh-they call; yugam-millennium; tat-jnah-the expert astronomers; yatra-wherein; dharmah-religion; vidhiyate-is performed.