तस्य शैलस्य that mountain's, दक्ष्णिपश्चिमाम् southwest side, कोटिम् end of, आसेदुः reached, तेषाम् for them, तत्रैव while being there, वसताम् while they stayed, सः कालः that time, व्यत्यवर्तत slipped.

They reached the end of the southwest side of the mountain(Vindhya). While they were still there, the time stipulated for them (to find Sita) had slipped.