The Law Code of Yājñavalkya 1.6.79

1.6.79

adhyetṝṇāṃ paśvādibhir antarāgamane kṛte śakradhvajasyāvaropaṇadivase ucchrāyadivase cāhorātram anadhyāyaḥ | dyuniśam iti prakṛte punar ahorātragrahaṇaṃ saṃdhyāgarjitanirghātabhūkampolkānipātaneṣv ākālikatvajñāpanārtham, ākālikanirghātabhūkamparāhudarśanolkāḥ (GDh 16.22) iti gautamavacanāt | nimittakālād ārabhyāparedyur yāvat sa eva kālas tāvat kālo 'kālaḥ tatra bhava ākāliko 'nadhyāyaḥ | etac ca prātaḥsaṃdhyāstanite | sāyaṃsaṃdhyāstanite tu rātrim eva, sāyaṃsaṃdhyāstanite tu rātriṃ prātaḥsaṃdhyāstanite 'horātram iti hārītasmaraṇāt | yat punar gautamenoktaṃ śvanakulasarpamaṇḍūkamārjārāṇām antarāgamane tryaham upavāsaḥ vipravāsaś ca (GDh 1.59) iti, tat prathamādhyayana eva || 1.147 ||

When an interruption is made by animals and the rest passing between Vedic reciters, on the day of lowering Indra's banner, and on the day of raising it, there is cessation of study for a day and night. Although 'day and night' was already under discussion, mentioning 'a day and night' again is to indicate that it lasts until the same time next day in the cases of twilight thunder, thunderbolts, earthquakes, and meteor showers, in accordance with Gautama's rule: 'Thunderbolts, earthquakes, eclipses, and meteors cause cessation lasting until the same time next day' (GDh 16.22). Beginning from the time of the occurrence, as long as the same time on the following day has not passed, that duration is an improper time; the cessation of study occurring then is ākālika (lasting until the same time next day). And this applies when thunder occurs at morning twilight; when thunder occurs at evening twilight, however, it lasts only for the night, because of Hārīta's traditional teaching: 'When thunder occurs at evening twilight, it lasts for the night; when thunder occurs at morning twilight, for a day and night.' As for what was stated by Gautama: 'If a dog, a mongoose, a snake, a frog, or a cat passes between, there is a three-day fast and dwelling away' (GDh 1.59), that applies only during first-time Vedic learning. || 1.147