Had some trouble Googling this little nugget, so I thought I might share it here.
If you want to constrain a date field in Open Data Kit by a specific date (i.e. “The date must be before 12/15/2000”), here’s the syntax:
<bind nodeset="/demographics/dob" type="date" constraint=". < date('2000-12-15')" jr:constraintMsg="Date of Birth must be before 12/15/2000" />
Don’t forget the quotes inside the parentheses.