In Section 13.2 make changes to BNF indicated in blue.

clocking_decl ::= [default] clocking [identifier] clocking_event ;

{ clocking_item }

endclocking

clocking_event ::= @ identifier

| @ ( event_expression )

event_expression ::= // This item is already defined in the BNF

cloking_item ::= default default_skew ;

| clocking_direction signal_or_assign_list ;

default_skew ::= input skew

| output skew

| input skew output skew

clocking_direction ::= input [ skew ]

| output [ skew ]

| input [ skew ] output [ skew ]

| inout

signal_or_assign_list ::= signal_or_assign { , signal_or_assign }

signal_or_assign ::= signal_identifier [ = hierarchical_expression ]

skew ::= edge [ # delay_expression ] // edge valid only if

| # delay_expression // clocking_event is simple edge

edge ::= posedge | negedge

delay_expression ::= constant_expression | time_literal

In Section 13.2 add the following sentence as the first sentence after the BNF.

The delay_expression must be either a time literal or a constant expression that evaluates to a positive integer value.