That wouldn't work well for people who replace the exe to update like me. That also does online time verses uptime which is what I'm going more for. So if your internet goes off the online timer stops until you reconnect. Since I personally have mirc open all the time my computer is running.
edit:
slightly better example of why not to check mirc.exe creation date. many people use already setup clients and the creation date would be whenever that was originally setup