Since JEvents 1.5.4 you can override the formatting of dates and times by changing the translations of various key translation strings (see http://www.jevents.net/jevents-15-topmenu/documentation/90-custom-overrides-of-language-strings-in-jevents for an explanation about how to customise parts of the JEvents translations).
In english the default translation strings are:
DATE_FORMAT_0="%A, %d %B %Y" which results in: Monday, 20 May 2013
DATE_FORMAT_1="%d %B %Y" which results in: 20 May 2013
DATE_FORMAT_2="%d %B" which results in: 20 May
DATE_FORMAT_3="%B %Y" which results in: May 2013
DATE_FORMAT_4="%d/%m/%Y" which results in: 20/05/2013
DATE_FORMAT_5="%d/%m" which results in: 20/05
DATE_FORMAT_6="%m/%Y" which results in: 05/2013
DATE_FORMAT_7="%A, %d" which results in: Monday, 20
TIME_FORMAT="%l:%M" which results in: 10:52
These use the strftime formatting mechanism (see http://php.net/manual/en/function.strftime.php) and to test your strings without making live changes you can use this useful site: http://strftime.onlinephpfunctions.com/
So if you want to change a specific date format - find the translation string that matches the current format and change it in your translation override file.
e.g. "Sunday, 07 January 2012" is from format DATE_FORMAT_0 so if you want this to read : "Sun: 7 Jan 2012" override the translation with "%a: %e %b %Y".