org-time-stamp-custom-formats

org-time-stamp-custom-formats is a variable defined in `org.el'.
Its value is

("<%m/%d/%y %a>" . "<%m/%d/%y %a %H:%M>")

Documentation:
Custom formats for time stamps. See `format-time-string' for the syntax.
These are overlaid over the default ISO format if the variable
`org-display-custom-times' is set. Time like %H:%M should be at the
end of the second format. The custom formats are also honored by export
commands, if custom time display is turned on at the time of export.

You can customize this variable.