The 6th-Case-affix is used in denoting location (adhikaraṇa) after a word denoting time (kāla) when used along with a word ending with an affix having the sense of kṛtvasuc -- saṃkhyāyāḥ kriyā'bhyāvṛttigaṇane kṛtvasuc [[5.4.17]] 'so many times'.,
Ex. पंचकृत्वोऽऽह्नोभुंक्ते 'he eats five times a day.' In short, 'words meaning so many times, or the numeral adverbs of frequency, govern the genitive of time in the sense of locative.' As द्विरह्नोऽधीते 'he studies twice in a day.'
Why do we say 'having the force of कृत्वसुच् ? Observe अह्नि शेते 'he sleeps in the day.'
This rule will not apply when the adverb of frequency is understood, not expressed (prayoga) as in अहनि भुक्तम्. Nor when the time is not meant, as द्विः कांस्यापात्र्यां भुंक्ते 'he eats in two brass vessels.' So also when location is not meant : as द्विरह्नो भुंक्ते. The शेष being understood we have द्विरहन्यधीते.,