Ellicott's Commentary for English Readers
(3) What time.—
Heb., yôm, apparently with same meaning as beyôm in Psalm 56:10, “in the day.”
I am afraid . . .—
No doubt the right reading: is, “I cry.”

MacLaren's Expositions
It is not given to many men to add new words to the vocabulary of religious emotion. But so far as an examination of the Old Testament avails, I find that David was the first that ever employed the word that is here translated, I will trust, with a religious meaning. It is found occasionally in earlier books of the Bible in different connections, never in regard to man’s relations to God, until the Poet-Psalmist laid his hand upon it, and consecrated it for all generations to express one of the deepest relations of man to his Father in heaven. And it is a favourite word of his. I find it occurs constantly in his psalms; twice as often, or nearly so, in the psalms attributed to David as in all the rest of the Psalter put together; and as I shall have occasion to show you in a moment, it is in itself a most significant and poetic word.

But, first of all, I ask you to notice how beautifully there comes out here the occasion of trust. ‘What time I am afraid, I will put my trust in Thee.’