Enhance RPG IV DATE FORMAT to include more clear 4-DIGIT Year Options

Extend RPG IV support for date format keywords in opcodes and built-in functions to include *MDYY, *YYMD, and *DMYY, enabling clearer intent and improving developer productivity when working with 4-digit year formats.

There is a growing need for additional date format keywords in RPG IV to support common and intuitive 4-digit year formats. While the language currently provides:

*MDY, *DMY, *YMD (2-digit year formats), and *CYMD, *CMDY, *CDMY (century-based formats),

It lacks three formats that developers frequently look for:

*MDYY (MM/DD/YYYY)

*DMYY (DD/MM/YYYY)

*YYMD (YYYY/MM/DD)

These formats are commonly required in modern applications where 4-digit years are now the standard, both for clarity and compliance reasons. Currently, developers are forced to consult documentation or experiment with less intuitive symbolic formats like *ISO, *USA, *EUR, or *JIS, each of which often requires an interruption to workflow to determine which one is needed This not only breaks the developer’s workflow and hampers productivity.

By introducing *MDYY, *YYMD, and *DMYY, RPG IV would allow developers to express their intent more clearly and work more efficiently, especially when handling user-facing date formats or converting between non-date fields and date field formats programmatically.

To be clear, this request is limited to supporting these additional keywords in opcodes and built-in functions (e.g., %date, %char, MOVE, MOVEL, etc.). While adding support in the DATFMT keyword on DCL-S and D specs would also be welcome, it is understood that this could introduce broader implications and is therefore not required as part of this enhancement. In other words, they would be the identifier that controls the non-date field's "date format" for conversions to and from dates.

This request builds on existing conventions already present in the language. Since support for formats like *CYMD has been established, this proposal should be relatively low-impact from a language design perspective while offering high value to the development community.