A timeline of the Vampire Diaries series has been created to resolve the inconsistencies between the original four books and those of The Return arc.
In Dark Reunion, Elena's year is said to be the "year of '92", which is supported by Bonnie's final diary entry. However, a large amount of the technology used in The Return, including mobile phones and the internet, was inexistent in 1992, but, most importantly, it is said to be the 21st century.
The year in which the first three novels took place was a common year beginning with a Tuesday, including a Friday 13 December. The next two possible years are 2002 and 2013. Given that the technology featured in The Return is not appropriate for 2002 neither, we must assume that the first three books take place in 2013.
For the sake of convenience, this dating system will be used by Founders' Archives for all Vampire Diaries articles.