For blanks you could use ToText({Your_Date_Field}) = "" though your DB may not allow spaces in date fields.
If your CR is set to change NULLs to default values, I was given this formula in this forum that works Length(ToText({Your_Date_Field}) < 2).
James P. Cottingham
When a man sits with a pretty girl for an hour, it seems like a minute. But let him sit on a hot stove for a minute and it's longer than any hour. That's relativity.
[tab][tab]Albert Einstein explaining his Theory of Relativity to a group of journalists.