BhG 8.17

syntax

[ye] (those who) brahmaṇaḥ (of Brahmā) sahasra-yuga-paryantam (whose extend is a thousand aeons) yat ahaḥ (which day) viduḥ (they know),
[ye] (those who) [brahmaṇaḥ] (of Brahmā) yuga-sahasrāntām (whose end is after a thousand aeons) [yat] rātrim (which night) [viduḥ] (they know),
te (they) janāḥ (people) aho-rātra-vidaḥ (knowers of day and night) [santi] (they are).

grammar

sahasra-yuga-paryantam	–	sahasra-yuga-paryanta 2n.1 m.; BV: yasya sahasraṁ yugāni paryanto ‘sti tat – whose extend is a thousand aeons (from: sahasra – a thousand; √yuj – to yoke, to join, to engage, yuga – aeon, an age of the world; pary-anta – circuit, edge, end, limit, boundary);
ahaḥ	–	ahar / ahan 2n.1 n. – day;
yat	–	yat sn. 2n.1 n. – which;
brahmaṇaḥ	–	brahman 6n.1 m. – of Brahmā (from: √bṛh – to increase);
viduḥ	–	√vid (to know, to understand) Perf. P 1v.3 – they got to know, they learned;
rātrim	–	rātri 2n.1 f. – night (from: √rā – to grant or √ram – to play, to rejoice);
yuga-sahasrāntām	–	yuga-sahasra-antā 2n.1 f.; BV: yasyā yugānām sahasreṇānto ‘sti tām – whose end is after a thousand aeons (from: √yuj – to yoke, to join, to engage, yuga – aeon, an age of the world; sahasra – a thousand; anta – the end, limit, boundary, death);
te	–	tat sn. 1n.3 m. – they;
aho-rātra-vidaḥ	–	aho-rātra-vit 1n.3 m.; ye ahar ca rārtiṁ ca vetti te – knowers of day and night (from: ahar / ahan – day; √rā – to grant or √ram – to play, to rejoice, rātra = rātri – night; √vid – to know, to understand, -vit – suffix: who knows);
janāḥ	–	jana 1n.3 m. – people, men (from: √jan – to be born, to produce);

textual variants

ahar yad → ahar ye (day, those who);
rātriṁ → rātrir (night);
yuga-sahasrāntāṁ → yuga-sahasrāṁte (in one whose end is after a thousand aeons);
te ‚ho-rātra-vido → te ‘ho-rātri-vido / aho-rātra-vido (they are the knowers of day and night / knowers of day and night);

Śāṃkara

brahma-loka-sahitā lokāḥ kasmāt punar-āvartinaḥ ? kāla-paricchinnatvāt | kathaṃ ?—

sahasra-yuga-paryantaṃ sahasrāṇi yugāni paryantaḥ paryavasānaṃ yasyāhnas tat ahaḥ sahasra-yuga-paryantam | brahmaṇaḥ prajāpater virājo viduḥ | rātrim api yuga-sahasrāntām ahaḥ-parimāṇām eva | ke vidur ity āha—te’ho-rātra-vidaḥ kāla-saṃkhyā-vido janā ity arthaḥ | yata evaṃ kāla-paricchinnās te, ataḥ punar-āvartino lokāḥ

Rāmānuja

commentary under the verse BhG 8.19

Śrīdhara

nanu ca tapasvino dāna-śīlā vīta-rāgās titikṣavaḥ | trailokya-sopari-sthānaṃ labhante loka-varjitam | ity ādi purāṇa-vākyais trailokyasya sakāśān maharlokādīnām utkṛṣṭatvaṃ gamyate | vināśitve ca sarveṣām avaśiṣṭe katham asau viśeṣaḥ syād ity āśaṅkya bahv-alpa-kāla-sthāyitva-nimitto ‚sau viśeṣa ity āśayena sva-mānena śata-varṣāyuṣo brahmaṇo ‚hany ahani trilokya utpattiḥ niśi niśi ca pralayo bhavatīti darśayiṣyan brahmaṇo ‚horātrayoḥ pramāṇam āha sahasreti |

sahasraṃ yugāni paryanto ‚vasānaṃ ysya tad brahmaṇo yad ahas tad ye viduḥ yuga-sahasram anto yasyās tāṃ rātriṃ ca yoga-balena ye vidus ta eva sarvajñā janā aho-rātra-vidaḥ | yeṣāṃ tu kevalaṃ candrāditya-gatyaiva jñānaṃ te tathāhorātra-vido na bhavanti | alpa-darśitvāt | yuga-śabdena atra caturyugam abhipretaṃ caturyuga-sahasraṃ tu brahmaṇo dinam ucyate iti viṣṇu-purāṇokteḥ | brahmaṇa iti ca mahar-lokādi-vāsinām upalakṣaṇārtham | tatrāyaṃ kāla-gaṇanā-prakāraḥ | manuṣyāṇāṃ yad varṣaṃ tad devānām aho-rātram | tādṛśair aho-rātraiḥ pakṣa-māsādi-kalpanayā dvādaśabhir varṣa-sahasraiś catur-yugaṃ bhavati | catur-yuga-sahasraṃ tu brahmaṇo dinam | tāvat parimāṇaiva rātris tādṛśair aho-rātraiḥ pakṣa-māsādi-krameṇa varṣa-śataṃa brahmaṇaḥ paramāyur iti

Madhusūdana

brahma-loka-sahitāḥ sarve lokāḥ punar āvartinaḥ | kasmāt ? kāla-paricchinnatvād ity āha sahasreti | manuṣya-parimāṇena sahasra-yuga-paryantaṃ sahasraṃ yugāni catur-yugāni paryanto ‚vasānaṃ yasya tat | caturyuga-sahasraṃ tu brahmaṇo dinam ucyate iti hi paurāṇikaṃ vacanam | tādṛśaṃ brahmaṇaḥ prajāpater ahar-dinaṃ yad ye viduḥ tathā rātriṃ yuga-sahasrāntāṃ caturyuga-sahasra-paryantāṃ ye vidur ity anuvartate te ‚horātra-vidas ta evāho-rātra-vido yogino janāḥ | ye tu candrārka-gatyaiva vidus te nāhorātra-vidaḥ svalpa-darśitvād ity abhiprāyaḥ

Viśvanātha

nanu amṛtaṃ kṣemam abhayaṃ trimūrdhno ‚dhāyi mūrdhasu (BhP 2.6.19) iti dvitīya-skandhoktyā keṣāṃcin mate brahma-lokasya abhayatva-śravaṇāt | sannyāsibhir api jagamiṣitatvāt tatratyānāṃ pāto na sambhāvyate ? maivaṃ | tal-loka-svāmino brahmaṇo ‚pi pātaḥ syāt kim utānyeṣām iti vyañjayann āha sahasra iti | sahasraṃ yugāni paryanto ‚vasānaṃ yasya tad brahmaṇo ‚har dinaṃ yad ye śāstrābhijñā vidur jānanti te ‚ho-rātra-vido janā rātrim api tasya yuga-sahasrāntāṃ viduḥ | tena tādṛśāho-rātraiḥ pakṣam āsādi-krameṇa varṣa-śataṃ brahmaṇaḥ paramāyur iti | etad-ante tasyāpi pāto na kasyacid vaiṣṇavasya tasya brahmaṇo mokṣaś ceti vyañjitam

Baladeva

svargādayaḥ satyāntāḥ sarve lokāḥ kāla-paricchinnatvād vinaśyantīti bhāvenāha sahasreti | yad ye brahmaṇaś caturmukhasyāhar dinaṃ nṛ-māṇena sahasra-yuga-paryantaṃ viduḥ catur-yuga-sahasraṃ tu brahmaṇo dinam ucyate iti smṛteḥ | sahasraṃ caturyugāni paryanto ‚vasānaṃ yasya tat | tasya rātriṃ ca caturyuga-sahasrāntāṃ vidus ta eva yogino janā aho-rātra-vido bhavanti | na tv anye candrārka-gati-vido mahar-lokādi-sthitānām upalakṣaṇam etat | ayam arthaḥ nṝṇāṃ varṣaṃ devānām aho-rātraṃ tādṛśair aho-rātraiḥ pakṣam āsādi-gaṇanayā dvādaśabhri varṣa-sahasraiś catur-yugaṃ catur-yugānāṃ sahasraṃ tu brahmaṇo dinaṃ rātriś ca tāvaty eva tādṛśaiś cāho-rātraiḥ pakṣādi-gaṇanayā varṣa-śataṃ tasya paramāyur iti | tad-ante tal-lokasya tad-vartināṃ ca vināśād āvṛttiḥ siddheti