First of all: my pet peeve, quoted from from the manual on .htaccess files:
You should avoid using .htaccess files completely if you have access to httpd main server config file. Using .htaccess files slows down your Apache http server. Any directive that you can include in a .htaccess file is better set in a Directory block, as it will have the same effect with better performance.
Second, more reading of the manual is also useful:
The IndexOrderDefault directive is used in combination with the FancyIndexing index option.
Translation: include the FancyIndexing option in the IndexOptions directive.
Then right below that in the manual:
IndexOrderDefault takes two arguments. The first must be either Ascending or Descending, indicating the direction of the sort. The second argument must be one of the keywords Name, Date, Size, or Description...
That results in the following:
<Directory /some/path>
# Disable .htaccess files for performance:
AllowOverride none
# Enable automatic index generation for directories without a DirectoryIndex file
# and sort them by date:
Options +Indexes
IndexOptions FancyIndexing
IndexOrderDefault Descending Date
</Directory>
Options -Indexescompletely disables automatic index generation...