IIS log files are usually based on GMT time. A log file is generated for each day from 00:00 GMT till 23:59 GMT.
The Windows Task Scheduler that runs the AWStats update uses the server local time.
ex%YY%MM%DD is calculated from the server local time.
To give a more specific answer, I need to know at what time the Task Scheduler runs the update and what time zone the server is using (i.e. where the server is located).
If you follow my recommendation, on July 8, 2010 at 5:00 am, AWStats will use the file named "ex100707.log". On July 9 at 5:00 am, it will use the file named "ex100708.log", and so on. And on August 1, it will use the file named "ex100731.log". I guess this is what you want ?