I haven't had much luck with dedicated dashcams bought through EBAY. The video quality has been pretty much unusable on a few I've tried from U.S. sellers. I now just use my Android Smartphone recording 720P HD using the free app
DailyRoads Voyager. Works extremely well, and records not just date/time, but can also display speed and GPS coordinates in the recorded video. Can also be setup to auto start and offers continuous recording, reusing the space occupied by old temp dashcam files unless you press the large red video button that saves that current file. Will also save the current video if it detects an impact.
There are a few draw backs to using a Smartphone as a dashcam. You need to find a suitable windshield mount that wont block the camera's lense. And stability can be an issue where road bumps and vibrations are also recorded in the dashcam video.
Plus, current Android devices cannot overlay dashcam display info over recorded video. However, you can view the recorded data on your computer by enabling CC(Closed Captioning) in your media player, such as VLC.
The developer offers a service to burn in the data on your dashcam videos for a small fee. But you can do it yourself fairly easily using a freeware video converter. When I get some time, I'll post some dashcam examples and how to use the converter.
In the meantime, you may want to search the "Technology Corner" for a couple of threads on dedicated dashcams recently.