This substitution of ahna for ahan does not take place after a numeral when it forms a collective noun meaning a number of days taken as a unity.,
This substitution is prohibited in the case of Samahara-Tatpurusha. Thus द्वे अहनी समाहृते = द्व्यहः, so also त्र्यहः ॥ Here rule 5.4.9 applies.
Why do we say \when meaning a collective noun?\ Observe द्वयोरह्नोर्भवो = द्व्यह्नः, त्र्यह्नः ॥ This is a Taddhitartha Dvigu compound formed by (II 1. 51); the affix अण् denoting तत्र भवः 4.3.53 is elided by 4.1.88: and the final अन् of अहन् is elided by 6.1.145.,