I'm guessing you are looking at Romans 16:25:
Τῷ δὲ δυναμένῳ ὑμᾶς στηρίξαι κατὰ τὸ εὐαγγέλιόν μου καὶ τὸ κήρυγμα Ἰησοῦ Χριστοῦ, κατὰ ἀποκάλυψιν μυστηρίου χρόνοις αἰωνίοις σεσιγημένου
and at Robertson's Word Pictures:
Through times eternal (χρονοις αιωνιοις). Associative instrumental case, "along with times eternal" (Robertson, Grammar, p. 527). See 1Co 2:6,7,10.
I don't find Robertson's description very clear for this verse.
This is a "dative of time", which can refer to either a point of time or a period of time. Robertson is saying this refers to a period of time in this case. Here's another good example:
καὶ χρόνῳ ἱκανῷ οὐκ ἐνεδύσατο ἱμάτιον - and for a long time he had worn no clothes (Luke 8:27)
If you just keep in mind that dative can be used for time, the context and the meaning of the words usually makes the rest clear. The exact relationship to time can vary, it can refer to a point in time, a period of time, or something that reoccurs over time, e.g. two verses after the last example, we find this:
πολλοῖς γὰρ χρόνοις συνηρπάκει αὐτόν - for many a time it had seized him (Luke 8:29)
On this point, I find Moulton clearer than Robertson, so I'm attaching a page from his grammar.
ἐξίσταντο δὲ πάντες καὶ διηποροῦντο, ἄλλος πρὸς ἄλλον λέγοντες, τί θέλει τοῦτο εἶναι;