A British CSV: it looks American, and the dates are the other way round
A British file is the American arrangement with one thing reversed. Commas between the fields, a full stop before the cents, grouped amounts in quotes — and dates written day first, 31/12/2026. Nothing in the file distinguishes it from an American one until a date runs past the twelfth of the month.
What a British file usually looks like
Saved by a spreadsheet set to English (United Kingdom), the file has a comma between the fields, a full stop as the decimal mark and a comma between thousands, so one thousand two hundred and thirty-four and a half is written 1,234.56. Dates are written DD/MM/YYYY: the last day of 2026 is 31/12/2026.
Because the thousands mark and the separator are the same character, a grouped amount only survives in quotes — the sample below writes 1,234.56 as "1,234.56", and a file that forgets the quotes gains a column on those rows.
None of this is a rule the file carries with it — there is no country written inside a CSV. It is what the program that wrote the file was set to, which is why every tool here reads the bytes and says what it found rather than trusting a flag.
Two conventions, one appearance
Set a British file and an American file side by side and, for any date in the first twelve days of a month, they are the same characters. 05/06/2026 is the fifth of June in one and the sixth of May in the other. There is no marker in a CSV that says which, so a spreadsheet or an import decides by its own regional setting — which is why the same file opened on two machines in two countries produces two different sets of dates, both without warning.
The tell is the same one in reverse. If any date in the column has a first number above twelve, the column is day-first and every ambiguous date in it can be read with confidence. If none does, the file genuinely cannot answer the question and the only reliable source is whoever exported it.
This is also why a British file that has been round a trip through an American tool can come back damaged in a way that is hard to spot: the dates that were ambiguous have been swapped, the dates that were not have been left alone, and the column now holds both conventions at once. A mixed column is the one case no pattern can rescue.
Reading them, and keeping them readable
The recipe on this page reads the date column with the day-first pattern and with the month-first one at the same time, so the rows where the two disagree are on screen rather than in the abstract. Run it on the sample first to see the shape, then drop your own file and change the table name in the query to the one the console gives it — the names are listed beside the editor — and the date column’s name too if your heading differs. Once the column is read correctly, writing it back in the international form — year, month, day — is what stops the question being asked again the next time the file is handed on.
The rest of a British file needs nothing done to it. Amounts use a full stop before the cents and group thousands with a comma, so a grouped amount sits in quotes to keep it in one field; that is ordinary, and every proper reader handles it. If some rows have gained a column, those are the rows where an amount above a thousand was written without its quotes, and the health check lists exactly those.
The sample file
Invoice,Customer,Date,Amount
INV-1001,Sample Supplies Ltd,31/12/2026,"1,234.56"
INV-1002,Example Trading Ltd,02/03/2026,89.50
INV-1003,Placeholder Limited,05/06/2026,32.10
INV-1004,Fictional Works Ltd,12/11/2026,"24,680.00"
INV-1005,Specimen Holdings Ltd,20/01/2026,410.00
Obviously made-up data, small enough to read. Download it and drop it on the tool to see the fix before trying your own file.
Questions
- Is 05/06/2026 the fifth of June or the sixth of May?
- In a British file, the fifth of June — the day comes first. The identical text in an American file is the sixth of May, and no part of the file itself says which was meant.
- How do I prove my file is day-first?
- Find a date whose first number is above twelve, such as 31/12/2026. One is enough to settle the whole column, because no month goes above twelve.
- My dates came back wrong after opening in Excel — can I undo it?
- Only if the column is still consistent. If the ambiguous dates were swapped and the rest were left alone, the column now mixes both conventions, and no single pattern can separate them again.
- What should I convert the dates to?
- The international form, year first: 2026-12-31. It sorts correctly as text, it cannot be read the wrong way round, and it survives being passed between countries unchanged.
- Why are some of my amounts in quotes?
- Because the comma that groups thousands is also the separator, so the quotes are what keep 1,234.56 in a single field. Without them the row gains a column.
- Does my file leave the browser to run the query?
- No. The query runs against the file inside this page on your own machine; click through the requests in the Network tab while it runs and none of them carries your file.