Heidegger's Bible Handbook: Galatians: Time of Writing
4. It appears to have been written near the end of the two years at Ephesus, a little before both Epistles to the Corinthians.
This Epistle is located among the first written by Saint Paul by Tertullian in his adversus Marcionem, book 1. By that very Paul, says he, who, being yet rude in grace, and even trembling, lest he should have run or were still running in vain, then for the first time conferred with the Apostles going before himself. Therefore, if he, being yet a neophyte, in fervor against Judaism, thought anything to be reprehended in their conduct, namely, their indiscriminate fellowship, afterwards himself becoming in his practice all things to all men, that he might gain all, you would have his censure of mere association, destined to become acceptable even to the accuser himself, to be suspect. That this Epistle was early written by Saint Paul not any of us doubts: but, that it was written as the first of all, is not so evident. For those written to the Thessalonians and to Titus appear to have preceded this. With these excepted, this Epistles appears to have been written before all the others near the end of the two years at Ephesus, both because in Galatians 6:10, he exhorts to beneficence towards the members of the household of faith (namely, those of Jerusalem, Romans 15:25, 26), and that, says he, ὡς καιρὸν ἔχομεν, as we have opportunity, that is, as long as this occasion is present, when Asia, Greece, and Macedonia appear to have already united for benevolence towards the Jerusalemites, and the collections received everywhere are to be carried to Jerusalem; whence also in 1 Corinthians 16:1 he instructs the Corinthians in making collections to follow the example of the Galatians; so that hence it might be rightly gathered that this Epistle was written a little before the first to the Corinthians: and because in Galatians 1:6 Saint Paul complains that the Galatians had been so quickly removed unto another Gospel. Now, they are appear to have been converted at that time, when Saint Paul, moving through τὰ ἀνωτερικὰ μέρη, the upper parts, namely, Phrygia and Galatia, proceeded to Ephesus, Acts 18:23; 19:1. At the very least, that the Galatians were already converted, when he wrote the Epistle to the Corinthians, is gathered from 1 Corinthians 16:1. Therefore, thus this Epistle would have been written about the year of our Lord 59, twenty-four years after the conversion of Paul. In which year the Most Learned Ussher also places this Epistle.
 Acts 19:1: “And it came to pass, that, while Apollos was at Corinth, Paul having passed through the upper coasts (τὰ ἀνωτερικὰ μέρη) came to Ephesus: and finding certain disciples…”