Excel changed the dates when I opened the CSV

You opened a CSV, and dates that read one way in the file now read another way on screen. Some rows look shifted by months. Saving from Excel has now written the new reading back to disk.

Why this happens

  • A CSV has no types. Every cell in it is text, so “04/08/2026” is eight characters and nothing more. Something has to decide what those characters mean, and when you double-click the file that something is Excel, using the date format of whatever machine you happen to be sitting at.
  • This is why the same file opens differently on two computers in the same office. A US locale reads 04/08 as 4 August; a UK one reads it as 8 April. Neither is wrong about the characters, and neither asked you.
  • The damage becomes permanent when you save. Excel writes back what it decided, so the original text is gone and there is nothing in the file recording that a conversion ever happened.

What to do right now

  • Do not double-click a CSV you intend to keep. In Excel, use Data → From Text/CSV, which opens the import dialog and lets you set the date column's locale before anything is parsed.
  • If the file has already been saved through Excel, go back to the original attachment rather than trying to repair the column. A shifted date and a correct date look identical, so there is no way to tell from the result which rows were changed.
  • Open it in a text editor first to see the real characters. That is the only view of a CSV nothing has interpreted for you.
  • If you control the sender, ask for ISO dates (2026-08-04). It is the one format no locale reads differently, and it costs them one setting.

Stopping it on the next file

A date field reads the whole column before it reads any cell, so a file is interpreted once rather than a row at a time. If the column contains both 25/12 and 04/08, the first is unambiguous and settles the reading for the second. If the column genuinely supports both readings, that is reported rather than guessed at, because a date nobody can be sure of is worth flagging and not worth silently picking.

The vendor price list template is a sheet with those rules already set, if you want to see the shape of one before building your own.

Other things that go wrong

My SKUs turned into scientific notationA column of product codes, barcodes, or order numbers now reads 1.24E+11. Widening the column does not help, and formatting it as a number gives you a different set of digits than you started with.The leading zeros disappeared from my spreadsheetProduct codes, zip codes, or account numbers that started with one or more zeros now start with a digit. 00123 is 123, and 07030 is 7030.The supplier sent the same SKU twiceOne product code appears on more than one row of the same file, sometimes with two different prices. Nothing in the file says which of them is current.My price column will not add upA column that plainly contains prices sums to zero, or sums to less than it should. Some cells are left-aligned where others are right-aligned.

Define the sheet once

Name your fields, and every file after that is matched to them and checked cell by cell before a row is allowed to land.

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