Excel may interpret dates differently depending on regional settings, file source and the way the values were imported. A date that is ambiguous, such as 03/04/2026, can be read as either 3 April or 4 March.
Start with the obvious checks
Check whether the cells contain genuine dates or text that only looks like dates. Set the correct regional format before importing data where possible. Use Data > Text to Columns or date conversion formulas when imported values have been interpreted incorrectly.
What else to check
For business spreadsheets, store real dates consistently and apply a display format such as dd/mm/yyyy. This separates the underlying date value from how it appears on screen.
When to get help
Send a sample and explain the format you expect.