In the Search Help section, under the section Dates, it says:
Dates can be entered in the following formats:
- year only – e.g., created:2012..2013 searches posts created from January 1, 2012 through December 31, 2012; [...]
This is similar to this question, but on a different piece of text in the same section.
As written, it seems to imply that the end range isn't included, that the range stops before it gets to 2013. I've tested that range; results from 2012 and 2013 are included.
Please correct the text. Change the text from:
January 1, 2012 through December 31, 2012
January 1, 2012 through December 31, 2013