The affix tumun comes after a root when the words 1. kāla 2. samaya 3. velā (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 (III 1. 113) 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.,