INTERVAL ' { integer | integer time_expr | time_expr } ' 
{ { DAY | HOUR | MINUTE } [( leading_precision )]) 
| SECOND [( leading_precision [, fractional_seconds_precision] )]
} 
[TO { DAY | HOUR | MINUTE | SECOND [( fractional_seconds_precision )] }]

For information on the symbols and conventions of Backus-Naur Form (BNF) syntax, see Appendix A.