अह्नोऽह्न एतेभ्यः

For अहन् is substituted अह्न when preceded by the above words (अहन् or सर्व or a word denoting a portion of night, or संख्यात or पुण्य) (and taking the affix टच् to be mentioned in राजाऽहस्सखिभ्यष्टच् 5.4.91).
When the affix टच् follows अहन्, and the latter is preceded by the words सर्व &c, then अह्न is substituted for अहन् ॥ By the term एतेभ्यः "these words", is meant the numerals (with the exception of एक 5.4.90) the Indeclinables, the words सर्व, संख्यात and words denoting a part of a day, for these are the words mentioned in the last sutra. The word अहन् is also mentioned therein, but it is not taken here, for there can be no Tatpurusha compound of अहन् with अहन् ॥ The word पुण्य is also excluded by 5.4.90. Thus द्वयोरह्नोर्भवः = द्व्यह्नः, त्र्यह्नः, अहरतिक्रान्तः = अत्यह्नः, निरह्नः ॥ So also सर्वाह्णः, 8.4.7, पूर्वाह्णः, अपराह्णः, संख्याताह्नः (according to some संख्याताहः 5.4.90).

The ordaining of substitution is redundant according to the Vartika kara. In his opinion, the sutra अह्नः एतेभ्यः alone would have been enough. The meaning then would have been "the affix अच् comes after the word अहन् when preceded by these words सर्व &c". The adding of अच् would produce the same result: as द्वि + अहन् + अच् = द्वि + अह्न् + अच् 6.4.134 the middle अ of अहन् is elided)) = द्व्यह्नः ॥ The only specific result which टच् would have produced was to make the feminine of these words end in long ई 4.1.15, and the elision of the final अन् of अह्न 6.4.145. But the compounds of अहन् are masculine always 2.4.29, they can have no feminine: and as the desired form is द्व्यह्नः, the sutra 6.4.145 cannot apply. The affix अच् is therefore, the proper affix and the substitution of अह्न for अहन् is redundant.