The "Start Time" column shown on Job History server's main webpage (http://<host>:<port>/jobhistory) is actually showing the submit time for jobs. However, when you drill down to an individual job's page, there the "Start Time" really does refer to when the job actually started.
This also true for the web services REST API, where the Jobs listing returns the submit times as "startTime", but the single Job API returns the start time as "startTime".
The two different times being referred to by the same name is confusing. However, it is useful to have both times, as the difference between the submit time and start time can show how long a job was stuck waiting in a queue. The column on the main job history page should be changed to "Submit Time" and the individual job's page should show both the submit time and start time. The web services REST API should be updated with these changes as well.