The ṇa is the substitute of na of ahan , when it is preceded by a word ending in a , having in it a letter capable of producing a change.,
Thus पूर्वाह्णः \fore-noon\; अपराह्णः \after-noon\.
Why do we say \ending in अ\? Observe, निरह्नः, दुरह्नः ॥
The word अह्न is substituted for अहन् by 5.4.88. The word अह्न being used in the sutra as ending in अ, the rule does not apply to other words. Thus दीर्घाह्नी शरत् ॥ The word अह्नः in the sutra is in the nominative case, and should not be construed as the genitive of अहन्, because all sthanins are exhibited in Nominatives as, वनं 8.4.4, वाहनं 8.4.8, यानं &c. The compounding takes place by 2.2.1, then is added the samasanta affix टच् 5.4.91, and अह्न substitution by 5.4.88. The word दीर्घाह्नी is a Bahuvrihi compound and not a Tatpurusha, and hence the affix टच् does not apply to it. The feminine ङीप् is added by 4.1.28, there is elision of अ of अहन् by 6.4.134.,