Use ISO8601 when exporting dates to CSV files
When exporting to a CSV file, it would be best to export dates in ISO8601 format (or a specific date format if the user has set one). CSV is about making things machine readable and this avoids the ambiguity of American vs British date formats. I regularly deal with local councils and they will export a CSV file from Excel and then be very confused as to why the date format has changed when they've explicitly set it. If you can't preserve their chosen format, please default to ISO8601 to reduce headaches.
Conal Tuohy commented
ISO8601 is absolutely the only sensible format for CSV
Will Deakin commented
I support this approach
Vinoth Sugunan commented
At this point once you open a file with ISO8601 dates in it the best you can hope for is that you have disabled the auto-save feature and Excell hasn't completely ruined your data. even if your date is inside a cell with words surrounding it, Excell will seek and destroy your properly formatted dates so that you can't easily convert them back.