sandhyā—transitional period before; sandhyā-aṁśayoḥ—and transitional period after; antaḥ—within; yaḥ—that which; kālaḥ—duration of time; śata-saṅkhyayoḥ—hundreds of years; tam eva—that period; āhuḥ—they call; yugam—millennium; tat-jñāḥ—the expert astronomers; yatra—wherein; dharmaḥ—religion; vidhīyate—is performed.