The affix ṭhañ comes after a word denoting time in the Instrumental -3rd case in construction in the sense of 'to be accomplished by that time'.,
Thus अह्ना निर्वृत्तं = आह्निकम् \to be accomplished in a day\ i. e. a certain portion of reading &c, आर्धमासिकम्, सांवत्सरिकम् ॥,