Related: http://groups.google.com/group/squeryl/browse_thread/thread/4efdc258dbd99e35

Currently the - operator between do dates is unsupported.
With some finagling you can get date subtraction to work and with Oracle and it will return a decimal value but I believe (this is untested) PostgreSQL uses an interval return type, and H2 returns a date diff in days so the implementation of this feature may be non-trivial.