This user's frustration is entirely justified, and the solution they are chasing does not exist inside Excel. No matter how many locale codes or custom formats you stack, a spreadsheet that must stay in xlsx and cannot use macros will always defer to the operating system's regional settings. That is not a bug. It is a design constraint baked into the tool itself. Excel treats date display as a presentation layer tethered to the local machine, because that is what desktop spreadsheet software has always done. The user is asking for a hardcoded date format. What they actually need is a different architecture.
The problem here is not the user's skill level. They tried the correct locale prefix, understood the regional setting issue, and correctly ruled out macros. The limitation is structural. A file format designed for local editing and manual entry cannot enforce a specific date string across every machine that opens it. The target system's requirement for "dd/mm/yyyy" is perfectly reasonable, but enforcing that inside an xlsx file without VBA is like trying to lock a door that was built without a lock. The workarounds that do exist, text-based date entry, data validation rules, helper columns, are brittle and rely on the user following instructions precisely. That is not a solution. It is a hope.
What this situation reveals is a gap between what spreadsheets promise and what they deliver when data must travel between systems. A user building a template for others should not have to guess whether a date will flip to "mm/dd/yyyy" when opened in a different region. The expectation that a format rule in the file should survive a machine change is not unreasonable. It is the tool that has not caught up. Modern data workflows demand that the format you set is the format that stays, regardless of who opens the file or where they sit. That means moving beyond the local-first model and toward a system where data is treated as portable, not parasitic on the viewer's settings.
If you are building templates that must feed into another system, consider whether the spreadsheet is the right container for that handoff. A simple text-based output, CSV or TSV with dates stored as plain text in the required pattern, will survive any regional setting because there is nothing to interpret. That is not a compromise. It is the correct architectural choice for data interchange. The user's problem is real, and the honest answer is that Excel will not fix it. The fix is to stop asking the spreadsheet to be a data gateway and start treating it as what it is: a flexible input tool that should hand off clean, uninterpreted text to the system that needs it.