I have been sent an Excel file which contains some dates in 2 columns. On visual inspection, the date values are correct in the sense that someone's date of birth matches a record we already have for them.
However, when I process the spreadsheet (using an ASP script but this also happens with a simple copy and paste to a new spreadsheet), the dates are all changed by 4 years and 1 day smaller.
The format of the column shows the expected DD/MM/YYYY (British English) format and locale - the file originated in the Republic of Ireland.
I'm at a loss as to how this can happen and how I can correct it for the future.