Thanks, Stephen, yes certainly, I can see that it could be a kind of present that continues into the future. Indeed, one or two have seen this for the perfect of ἐνίστημι - if something has 'set in', then it is likely to continue. With regard to Galatians 1.4 - ὅπως ἐξέληται ἡμᾶς ἐκ τοῦ αἰῶνος τοῦ ἐνεστῶτος πονηροῦ - Ellicott distinguishes between:
a) ὁ νῦν αἰὼν .. [and] b) .. 'the commencing age' .. which had already begun;
Likewise, Meyer:
Certainly in practical meaning ἐνεστώς may denote present (hence in the grammarians, ὁ ἐνεστὼς χρόνος, tempus praesens), but always only with the definite reference suggested by the literal signification, setting in, that is, in the course of entrance, that which has already begun.
Either way, it refers to something already present.
BDAG has categories 1) 'to take place as an event'; 2) 'to be present as condition or thing at the time of speaking'; 3) 'to be about to occur, with connotation of threatening'. Cited under 3 is:
ἀλλ' ἐνίστασθαι τοῖς Καρχηδονίοις [Polybius, histories, 3.97.1]
'but to keep up the pressure on the Carthaginians' [Loeb]; 'press hard' [BDAG]
It seems to me that if something is so impending that you can actually feel the pressure of it, then it is already here.
τῆς σκηνοπηγίας ἑορτῆς ἐνστάσης [Josephus, Antiquities 4.209]
'at the season of the feast of tabernacles' [Loeb]
I don't see what this is doing under category 3..
ἐὰν γὰρ περὶ τῶν ἐνεστώτων ἢ μελλόντων γράφω ὑμῖν, [Letter of Barnabas 17.2]
'For if I should write to you concerning things immediate or future,' [Lightfoot]
Nor especially this, where we have the perfect tense ..
Andrew