For Orders in BA database, order detail end date is stored in UNIX timestamp, and the detail end date is usually set not to 01 Jan 2010 23:59:59, but to 02Jan 2010 00:00:00 - these dates are just an example.
In Control Panel the detail end date is shown as 01 Jan, meaning that the detail is active for the whole 01 Jan day.
However when OrderFinDetailsListGetExt_API request is sent, it returns actual day of the Unix timestamp, which is 02 Jan in this example.
Documentation bug has been submitted: PBA-79723 (OrderFinDetailsListGetExt_API returns EndDate as the next day).
Provider needs to consider this behavior when using the OrderFinDetailsListGetExt_API call.