After the words ṣaṣṭhī etc. when no other Numeral precedes them; tamaṭ is always the augment of ḍaṭ,
By sutra 5.2.56, the augment was optional, this makes it compulsary. Thus षष्टितमः, सप्ततितमः ॥
Why do we say \when not preceded by a Numeral\? Observe, एकषष्टः एकषष्टितमः, एकसप्तमः or एकसप्ततितमः ॥ Here 5.2.56, applies also.,