Just found that if you set the ’System configuration’->’Interface & Features’ ’Date format’ to dd-mm-YYYY you will get some odd results when revisiting date fields.
Basically the javascript behind datepicker does not parse the dd-mm-YYYY format of the date correctly and attempts to get the date as the month and month as the date. It means for example if you reopen a resource and go to change the date (say publish date) you will likely not see the default value in the datepicker that was in the date field.
Bug added here:
http://svn.modxcms.com/jira/browse/MODX-1462
