Quite closely relate to expression of existence / non-existence are expressions of past time, which make use of the phrase est cum and fuit cum, which may be translated as ‘there was a time when…’ Like the phrases that describe existence, these make use of a relative clause of characteristic (w/ subjunctive!) to describe an indefinite period of present/past time.
est cum …. present subjunctive
fuit cum …. imperfect subjunctive
est cum in omnis virī aevō domum parentis linquat: there comes a time in every man’s life when he must leave the home of his father
est cum omnibus deceat: there is a season for all things
fuit cum mihi quoque initium requiēscendī fore iūstum arbitrārer: there was a time when I thought a beginning of rest would be justifiable on my part
fuit cum nōn altior meīs genibus essēs: there was a time when you were no taller than my knees
Most grammar textbooks will tell you that the Latin ‘to be’ has only a future active participle. On a practical level, that’s true. However, there is evidence within the Latin language of a lost present active participle. This would have been sōns, sontis. (cf. Greek ὤν).
However, this form is all but lost. We may conjecture that it existed at one time because it is stored in certain adjectives (īnsōns, innocent; absēns, absent, praesēns, present). It also appears in late Latin philosophical terminology (ēns, being; entia, the things which are). However, these were likely designed by intellectuals to reflect the present participle as it would appear, were it in use. Honestly, the same might be true of insōns, etc, but with words that old, we can’t trace their origins properly.