Excel

Hi JP — thanks, I can help. Your description (works for the first ~22 rows then “reverts” to US-style) sounds like a formatting / locale detection issue in Excel rather than a bug in your typing. A few different things can cause that behavior depending on whether you type the dates, paste them, or import a CSV. Below I give short explanations and specific step-by-step fixes for the common situations. Try the ones that match how you create the dates and tell me what you see.
Quick checks first

1. Are you typing the dates directly into Excel, or pasting/importing them from another program or a CSV file?
2. Which Excel version (e.g., Excel for Microsoft 365, Excel 2019) and Windows 11?
3. If you can, try this quick test: in a new blank workbook select column A, format it (steps below) to dd-mm-yy, then type 01-01-25, 02-01-25 … down 30 rows. Do the first 22 remain dd-mm-yy and the rest change? Tell me the result.

Solutions (pick the one that matches your situation)
A — If you are typing dates manually

1. Select the whole column where you enter dates (click the column header).
2. Press Ctrl+1 → Number tab → Category = Date. In the “Locale (location)” dropdown choose French (France) or French (Canada) depending on which French format you want. Pick the dd-mm-yy style. Click OK.
   • Or use Custom and enter: dd-mm-yy (or dd-mm-yyyy).
3. Now enter your dates. Excel will store them as dates and display them in dd-mm-yy for the entire column.

B — If some entries are stored as text or mixed types (causes formatting to not apply)

1. Select the column. Data → Text to Columns → Delimited → Next → Next. On the last screen set Column data format = Date and choose DMY from the dropdown → Finish.
   • This converts text-formatted dates to real Excel dates using day-month-year order.
2. Then apply the custom date format (Ctrl+1 → Custom → dd-mm-yy).

C — If you’re pasting from another app or importing a CSV
Excel (or Power Query) often “guesses” the column data type by sampling the first rows. If the sample contains only dd-mm entries Excel may treat later ambiguous entries differently.
If opening a CSV by double-click:

• Instead of double-clicking the CSV, use Data → Get Data → From Text/CSV → select the file. In that import window pick “File Origin” and in the preview step set the Locale/Region to French (or choose “Load with Power Query”). In Power Query Editor right‑click the date column → Change Type → Using Locale → choose Date and Locale = French. Then Close & Load.

If you paste:

• Paste into a column already formatted as Date (see A), or paste into Notepad first and then copy → paste into Excel to avoid hidden formatting bringing US-style dates.

D — If you are using an Excel Table

• If your range is an Excel Table (Insert → Table), set the format on the column header cell (Ctrl+1) — the Table should propagate the format to the new rows automatically. If some rows are already text, convert them first (use Text to Columns as in B).

E — Office / Windows locale settings (if Excel keeps ignoring your choice)

• In Excel: File → Options → Language. Make sure French is listed as an Editing language and (optionally) set it as default.
• In Windows 11: Settings → Time & language → Language & region → set Regional format to French (Canada) or French (France) and check Short date format. Changing Windows region can affect Excel’s interpretation of ambiguous dates.

Why “about 22 rows” might happen

• Some import/data-detection tools sample only the first N rows to decide a column type. If the first N rows all look like a certain format Excel/Power Query may set the column type and then later rows that don’t match get interpreted differently. That’s why forcing the column format (select column → Ctrl+1) or using Text to Columns / Change Type Using Locale fixes it.

If the above doesn’t fix it

• Tell me: do you type, paste, or import? Share the exact string you type for dates (example: 1/2/25 or 01-02-25). I can then give the precise steps. If you’re comfortable attaching a tiny sample workbook or a screenshot (no private data), attach it and I’ll look — note: I didn’t receive any attachment with your post, so please try uploading again if you meant to attach something.

Safety tip

• Make a copy of your worksheet before doing bulk conversions (Text to Columns, power-query loads), so you can revert if the result isn’t as expected.

If you want, I can give the exact step-by-step to import a CSV with French dates or the exact menu clicks for your Excel version — tell me which scenario and which Excel version and I’ll walk you through it.