org-html-metadata-timestamp-format

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

"%Y-%m-%d %a %H:%M"

Documentation:
Format used for timestamps in preamble, postamble and metadata.
See `format-time-string' for more information on its components.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.