I have looked at this. I am not a solicitor. I think the only relevance this has in Irish law is where this this clause is inserted into a legally binding document and the definition of "on or about" is made clear. When I say a legally binding document I mean a contract, an insurance agreement, a loan agreement from a bank. I stand to be corrected.
"The ICC rules of practice are designed by bankers and merchants and not by legislatures with political and local considerations. The rules accordingly demonstrate the needs, customs and practices of business. Because the rules are incorporated voluntarily into contracts, the rules are flexible while providing a stable base for international review, including judicial scrutiny".
The UPC 600 seems not to be a law itself, but is a guideline that can aid clarity and structure when drawing up legally binding agreements for financial instruments, such as letters of credit.