Good question. I’d say no (not necesarily). Right now, the parameter-less ToString() (which gets hit here, when passing objects to String.Format) calls the ToString (equivalent) of the underlying native date type, and thinking about it, I believe this to be the desired behavior.
Dates are a complex thing, and calling ToString() without a format for anything but debugging purposes seems like a really bad idea. To show dates to the end-user of your application, to save them to text files, or to in any other way, shape, or form get a consistent and reliable string representation for a date, you should call one of the ToString() overloads that take an explicit format, or cal the other To*String convenience methods, such as ToShortPrettyDateString, etc. (those should all behave consistently, and if they don’t, please let me know).
PS, missed that, but I assume you are only referring to the difference formatting, and that the actual date value was correct and you did run these two, some 88 seconds apart, yes?