Details matter here so please show the code used to generate the output files.
A CSV file is just text. The only metadata it can contain is the header row which is normally used as the names to use for the variables. In general SAS will use the attached formats when writing data to text, so if you have a DTDATE9 format attached to your datetime variables then the text written to the CSV file will look like ddMONyyyy. Note that if you had a character variable instead of a datetime variable and the values also looked like ddMONyyyy then what is written into the CSV file will look exactly the same. How the program that reads the CSV files determines what to do with that you will have to investigate more.
An XLSX file actual has structure, so it can have metadata about what types of values it contains. But what code you use to write the XLSX file can make a big difference about what metadata SAS creates. For example you can use STYLE option on PROC REPORT and PROC PRINT to control what formatting metadata is stored in the XLSX file. So you could, if you want, store the actual datateime values and tell Excel to display it in ddMONyyyy style that matches how the DTDATE9 format in SAS would display that datetime value.
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