org-export-date-timestamp-format

org-export-date-timestamp-format is a variable defined in `ox.el'.
Its value is

nil

Documentation:
Time-stamp format string to use for DATE keyword.

The format string, when specified, only applies if date consists
in a single time-stamp. Otherwise its value will be ignored.

See `format-time-string' for details on how to build this
string.

You can customize this variable.