Course Start Date
This is where you specify the starting time of the course (in your own timezone).
If you are using a 'weekly' course format, this will affect the display of the weeks. The first week will start on the date you set here.
This setting will not affect courses using the 'social' or 'topics' formats.
However, one place this setting will affect is the display of logs, which use this date as the earliest possible date you can display.
In general, if your course does have a real starting date then it makes sense to set this date to that, no matter what course formats you are using.