Entering date format as dd/mm and reverts to dd-mmm format how do I stop this?

I have been using spreadsheets since before Excel was born, starting back in 1984. I get paid to produce workbooks full of complex formulae and VBA code. My point - I am not a newbie to Excel. This is something I have not seen before and have run out of ideas to sort it out.

I have a set of fields that will be updated programmatically (via VBA) with the date that an entry in that column was last changed. The date will be used in a PDF report in the footer. I can change the date to the format that I require for the report by VBA. However, because my workbooks are used in many locations, I require the date in the spreadsheet to be in the format "d mmmm yyyy" to avoid any confusion. I have formatted the cells with this custom setting, but when I enter a date as "1 Mar 2019", which normally (in all my other spreadsheets) gets reformatted to "1 March 2019", this spreadsheet is formatting it as "03/01/2019" and displaying it in the spreadsheet in that format. In the UK this is 3rd January, not 1st March. Thinking this was something to do with the difference between UK date and US date formats, I manually changed the entry to "01/03/2019" and it changed it back again. However, when I put the date in as "03/01/2019" the spreadsheet changed it to "01/03/2019", which is what I want, but I have no confidence what that date means. The spreadsheet toggles whatever date I put into the opposite format, transposing the "dd" and "mm" figures. As this information will be produced programmatically in the future and I won't be around to supervise these entries, I have no confidence in this spreadsheet producing the correct result in the future. I have tried many, many things to sort this and come to you as a last resort. My next step is to decide that this spreadsheet is corrupted and to shred it, burn the shreddings, scatter the ashes and dance on its grave, laughing (the pressure is getting to me a bit!).