घकालतनेषु कालनाम्नः

The Locative -7th case-affix ending is optionally retained after a word denoting time ending in a consonant or अ when तरप् or तमप् or the word काल or the affix तन follow.
The affixes त॒र॒ and त॒म॒ are called घ 1.1.22. Thus घ :- पूर्वाह्णतरे or पूर्वाह्णेतरे, पूर्वाह्णेतमे or पूर्वाह्णतमे ॥ काल :- पूर्वाह्णेकाले or पूर्वाह्णकाले ॥ तन :- पूर्वाह्णेतने or पूर्वाह्णतमे ॥ Why do we say 'after a time-name'? Observe शुक्लतरे, शुक्लतमे ॥ The condition that the preceding word should end in a consonant or अ applies here also. Thus no option is allowed in रात्रितरायाम् ॥

As a general maxim, an affix denotes whenever it is employed in Grammar a word-form which begins with that to which that affix has been added, and ends with the affix itself (प्रत्ययग्रहणे यस्मात् स विहितस्तदादे स्तदन्तस्य ग्रहणम्) ॥ Thus the word थ, अथ &c in 6.2.144 means a word ending in थ affix &c. But in this chapter, so far as the jurisdiction of उत्तरपदे goes, an affix does not denote a word-form ending in that affix, on the following maxim : उत्तरपदाधिकारे प्रत्ययग्रहणे न तदन्त ग्रहणम्; on the contrary the affix denotes its own-form. Thus तर, तम and तन here do not denote a word ending in these affixes. This rule we infer from the fact that in sutra 6.3.50, the author declares "हृद् is the substitute of हृदय when the word लेख, and the affixes यत्, अण्, and लास follow". Had the affix अण् here meant the word-form ending in अण्, then there would have been no necessity of using the word लेख in the sutra, as लेख is formed with the अण् affix. The word काल in the aphorism means the word-form काल ॥ See Sutra 4.3.23 for the affix तन ॥