interval_value_exp

interval_value_exp 
      ::= 
      interval_term 
      | interval_value_exp_1 "+" interval_term_1 
      | interval_value_exp_1 "-" interval_term_1 
      | "(" datetime_value_exp "-" 
      datetime_term ")" interval_qualifier 

This rule is called by

-> datetime_value_exp
-> extract_source
-> interval_value_exp_1
-> set_time_zone_value
-> time_zone_specifier
-> value_exp