Date transfer from csv to xlsx incorrect

Hi there:

I’m using Ubuntu 16.04 LTS (64 Bit) on Lenovo Thinkpad T410. I’m not a tekkie and english is not my mother tongue, I apologize for any mistakes…

I’m constantly having this problem: I frequently do a download of csv files and copy / paste the tables into a xlsx file (which is stored on Dropbox for staff use). The dates are transferred incorrectly, the discrepancy seems to correspond to the example date shown in the formatting application (csv: dec. 31st 1999, xlsx: jan. 1st 2004). All the dates show with a discrepancy of 4 years, 1 day.

What can I do?
Thank you so much for your help!

I am not an Excel user but I believe this is the area you need to look at.
LibreOffice Calc uses the Gregorian calendar as the default. If you use Excel formats that use the Microsoft calendar, you need to tell LibreOffice calc the correct date adjustment to make. It is one of the longest standing uncorrected bugs. (Ms forgot that there was no leap year in 1900) I believe the Apple version of excel starts at 1 March 1904. Hence you need to set the first day of the 20th century. In calc choose Tools > Options > LibreOffice Calc > Calculate > Date > select 01/01/1904. Slightly bizarre this problem is still around.
Easiest option is to use Calc and .ods. As far as I know CSV files do not contain the date format used,