The following is found also in the "Author" section of 1 Timothy and Titus.
The three Pastoral Epistles (1 & 2 Timothy and Titus) have often been viewed as distinct from other letters in the Pauline Corpus. Marcion, the second Century heretic did not include the Pastorals in his accepted Canon, and there is very limited evidence that the Pastorals may not have been included in the Codex of the Pauline Epistles (P46), dated around the middle of the 3rd Century. However, Marcion's list was by no means an indication of the Church's view (he was, after all, a heretic and did not accept Matthew, Mark, John or any of the Jewish writings of the NT in his list), and P46 may have contained the Pastorals, but we do not know as the last pages are missing!
From the pages of history then, we can only see that in general the Pastorals were not questioned as to their Pauline authorship until the 19th Century. At this time, questions were raised over several issues: Historical problems, doctrinal problems, problems with content in general and linguistic issues.
The historical problems centered around a seeming inability for Paul to have written these letters, containing the information that they do about the authors' past journeys (for he left Titus in Crete [Titus 1:5]), which seem not to fit with the information recorded in Acts. There are several theories to explain this, and as such it seems that time is better spent exploring other issues of contention.
As was a point of objection concerning the letter to the Ephesians, opponents of Pauline authorship point to a lack of Pauline themes in the Pastoral epistles, and also to a large number of words used in the Pastorals but not used anywhere else in the Pauline writings, to evidence their claims. As with the reasoning in the authorship section of the letter to the Ephesians, I will point out that the letters of Paul were written sometimes to individuals (as is the case with the Pastorals), sometimes to a whole church (as is the case with most of his other letters), or possibly even to a group of churches (as may be the case with Ephesians). The letters were also written with different purposes, covering different themes, with different emphases. There is no reason for Paul to include 'classic Pauline themes' in every letter he writes, what would be the point? We must expect that the different letters, written to the different audiences, with different purposes should use different vocabulary.
The Pastorals are a good example of this: because the difference in content of these letters (focusing on pastoral issues such as church leadership) as opposed to other Paulines was noticed, the three were collectively called the Pastoral Epistles. So the content of these epistles set them aside as distinct in themselves from the other Paulines. The fact that there are a relatively high number of words used in these but not used elsewhere would be expected!
Finally, opponents of Pauline authorship may suggest that, based on other Pauline writings, he had no real interest in Church structure, especially not the extent as found in the Pastorals. However, firstly, it is clear that Paul and Barnabas had appointed elders earlier (Acts 14:23), and secondly, it is very unlikely that the apostle, led by the Spirit of God, had no thought of preserving sound teaching in an organised way.
It seems then, that though the Pastorals are clearly different in many ways from other Pauline writings, there is no substantial reason to doubt the claims of each letter, that they were written by Paul the Apostle.
Between 66-68 AD, depending on the date of the death of Paul.