Your answer will depend on how the data is entered to the file: it may
be a text entry, or it may be a numeric entry with the formatting set
to show a date.

Excel uses an integer number of days since a particular start date to
keep track of dates, and a fractional portion of a day to represent
time during the day. Try it: in an empty cell type =TODAY() and Excel
enters the number 39371 for 16 October 2007. You may see that number
or a date, depending on how the cell is formatted. If you right click
that cell and select Format Cells you can format it as a date in a
variety of formats.

In a different cell type the formula =NOW() and again the format of
the cell will determine how the decimal number is displayed. Right
click this cell and select a number display that contains 8 or 10
decimal places and you'll see the number 39371 to the left of the
decimal place (representing the date); the fraction represents the
time.

So back to reality: if your data is stored as Excel dates you can
simply format the cells to your desired output and your CSV file is
good to go. If the dates are stored as text, then you'll need to
convert the text dates to Excel dates, then reformat.