Date format shouldn't matter. Nor should blanks. The answer you are getting suggests you are getting a really low number that is then formatted as a date. Can you change the date format to include the year? I'd bet your answer is Jan 10/1900.

So the next question is why? I just ran a test of three dates and a blank cell, and the answer was correctly formatted to a date somewhat in the middle of the three. "Sept 15, Sept 20, blank, Oct 1" resulted in an average of Sept 22. This makes me think there is something strange in how you have your data set up. Is it possible Excel isn't seeing a true date?