It has always bothered me that in Windows Vista and Windows XP that the only way you can have the date shown by default is to have a multi-row task bar. (This is not an issue in Windows 7 since you are essentially forced to use a multi-row task bar.) After all these years, I am happy to say that I finally find a freeware to do this:
So, I prefer my dates to have the yyyymmdd format, but the program allows you to have other formats.
The program is pretty old, and the author’s webpage at http://users.iafrica.com/d/da/dalen has long since disappeared. However, you can still download the program from sites like PCWorld (http://www.pcworld.com/product/946967/tclockex.html).