The affix ṭhañ comes after a word of time in the sense of 'an abstinence that lasts so long' or 'who practises abstinence so long'.,
The word तद् shows that the word must be in the accusative case. अस्य shows the force of the affix, ब्रह्मचर्य्यः joins both. Thus मासं ब्रह्मचर्य्यमस्य = मासिको ब्रह्मचारी \who practises abstinence for a month\. So also आर्धमासिकः, सांवत्सरिकः ॥
Another explanation is, the affix than comes after a word in the first case in construction, when the sense is that of an abstinence which lasts for such a period. As मासोऽस्य ब्रह्मचर्यस्य = मासिकं ब्रह्मचर्यम् \an abstinence lasting for a month\. आर्धमासिकं, सांवत्सरिकं ॥
In the first case the affix relates to a person, in the second, to the vow itself. Both these explanations are valid, as the structure of the aphorism is open to both constructions.
Vart:- So also after the words महानाम्नी &c in the genitive construction. As महानामिकम्, गौदानिकम्, आदित्यव्रतिकम् ॥
Vart:- After those words, the affix means also 'who practises that'. As महानाम्नीश्चरति = माहानामिकः, आदित्यव्रतिकः, गौदानिकः ॥ महानाम्नी + ठञ् = महानामन् + ठञ् (6.3.35 Vart) = माहानामिकः 6.4.144. महनाम्न्य ऋचः, तत् सहचरितं व्रतं ॥
Vart:- The affix डिनि comes after the words अदान्तरदीक्षा &c, in the sense of 'who practices'. As अवान्तरदीक्षांचरति = अवान्तरदीक्षी, तिलव्रतिन् ॥
Vart:- The affixes ड्वुन् and डिनि come after the word अष्टचत्वारिंशत्, as अष्टचत्वारिंशद्वर्षाणि व्रतं चरति = अष्टचत्वारिंशकः or अष्टचत्वारिंशिन् ॥
Vart:- The same affixes come after the words चतुर्मास्य &c, the final य being elided. As चतुर्मास्यानि चरति = चातुर्मासकः or चातुर्मासिन् ॥
Vart:- The affix ण्य comes after चतुर्मास, in the sense of 'produced there'. As चतुर्षु मासेषु भवानि = चातुर्मास्यानि ॥ It refers to a sacrifice.
Vart:- The affix अण् comes, when it is a Name, as, चतुर्षु मासेषु भवा = चातुर्मासी पौर्णमासी ॥ So also आषाढी, कार्तिकी, फाल्गुनी ॥,