You can't do this with a formula.  You need to have something like a calendar table that contains all of the dates in a year.  You then use that as the "base" table for your report and do a left join from that to the table that contains the data with the dates.  Use the date field from the calendar table on your report instead of the one from the data table.

What type of database are you using?  This may be your database's internal representation of a blank date value.  Have you tried adding 1/1/2001 to your spreadsheet?  How about a blank field?