3.3.167 कालसमयवेलासु तुमुन्

Word by word (padaccheda)

Right context काल- समय- वेलासु saptamī · bahuvacanaSubstitute / term तु prathamā · ekavacana

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

काल-समय-वेलासु तुमुन्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

The affix तुमुन् comes after a root when the words 1. काल 2. समय 3. वेला (all meaning time) are in composition.
Thus कालो भोक्तुम् 'time to eat'; समयः खलु स्नानभोजने सेवितुम् 'it is time to bathe and take food'; वेला भोक्तुम् 'time to eat'. In short, Infinitive in तुमुन् may be used with words meaning 'time'; as अवसरोऽयमात्मानं प्रकाशयितुम् 'this is, indeed, the time to show myself'.

Why do not we use the Infinitive here? कालः पचति भूतानि 'time devours all creatures'. The sense of 'direction &c'. is understood in the sutra; so when 'direction &c' is not meant, the Infinitive will not be used.

Why do not we use the Infinitive here, कालो भोजनस्य 'proper time to eat'? Because, by the rule of non-uniform affixes 3.1.94, we can use also the affix ल्युट् (by which the word भोजन is formed). As we have already in sutra 3.3.163 that after sutras 2.1.11 and forward, namely, in other sutras than those treating of feminine affixes, the rule of 3.1.94 may be employed as an Anitya or a non-universal rule.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.