There are several ways to go about this:
1) use the date formatting
native to Ditto and use the +date+ placeholder in your ditto tpl (note need to use the dateSource parameter as well to define where to get the date);
2)
use PHx within your ditto Tpl to format the date field (phx is installed for use within Ditto as part of ditto);
3) use a snippet, like the dateTime snippet to format the date.
In MODx the dates are stored as a unix timestamp and you can format the date as you normally would in php using the stftime() or the date() functions.