I've did a quick test in Oracle 11g, and it forbids such interval values. PostgreSQL 9.4 forbids such intervals as well.
Should we drop PLUS/MINUS support for interval literals? A quick glance at the code shows inconsistent views on the usage of the sign. For instance: org.apache.calcite.sql.SqlIntervalLiteral.IntervalValue#signum vs org.apache.calcite.sql.fun.SqlMonotonicBinaryOperator#signum Vladimir
