In excel the ddd is the number of the day since the beginning of the year. For example, 02/15/18 entered in libreoffice calc displays as 02/046/18 in excel 2007-2013 XML (.xlsx). The user defined format code in the format cells dialog is mm/dd/yy;@
How do you get the ddd to display as the day of the current month? In the example above, we want the date to display in excel as 02/15/18
What version of LibreOffice do you use?
ddd in Excel is not the number of days since the beginning of year, it is the day of week short name, the same as in Calc, see also this support article.
AFAIK there is no format code to display days since the beginning of the year.
Which LibreOffice version do you use? AFAIK no version saves mm/dd/yy;@ format as mm/ddd/yy;@
It is version 5.4.3.2 (x64)
Format code is saved as MM/DD/YY;@ so whatever your Excel does (if it really switches to a ddd instead of dd and displays it as day of year, which I still doubt) is not what is in the file. At least not if such sample is created afresh. Maybe attach or put your file (cleaned of sensitive data) on a web space somewhere and place the URL here so we can take a look.
This is not an answer to the question.