Sounds like a legitmate problem report.
I support a lot of BlackBerry end users, and I never receive tickets about a BlackBerry losing time. I realize it happens, but I would have to categorize it as a rarely heard issue.
The reason most computers (handhelds included) lose time is that the battery is no longer holding enough of a charge. You might try buying a new BlackBerry battery to see if that helps. If that doesn't help, then you may have to return the entire handheld to Cingular.