12 August, 2013
The screenshot shows the string of the latest build version, this is listed as "9478.0.winblue_rtm_130804-1505". The date of compilation of the build "4 August 2013" specified
This does not mean that the RTM version has actually been finalized, but only that this build has reached the so-called "RTM Branch". This basically means that Build 9478 is still such a thing as a (late) Beta version. However, with all the builds that are referred to as RTM Branch, the possibility that they are the final RTM. In short: Signs are that Windows 8.1 is nearing completion.