After the words śata etc. and after 1. māsa 2. ardhamāsa and 3. saṃvatsara tamaṭ (tama feminine in ṅīp) is always the augment of ḍaṭ,
The words शतादय are numerals. The words मास &c, are not numerals; they would not have taken डट्, but by the implication of this sutra. Thus शतस्य पूरणः = शततमः 'hundreth'; सहस्रतमः, लक्षतमः ॥ Similarly मासस्य पूरणः = मासतमो दिवसः 'the last day of the month'. अर्द्धमासतमः, संवत्सरतमः ॥
By the next Sutra 5.2.58, the words शत &c, would also have taken तमट्; but the scope of that sutra is confined to numerals not preceded by another numeral. There is no such restriction here. Hence we have एकशततमः, द्विशततमः ॥,