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) ]
     }
]