To get a date that you can use, use the util.scand method: http://livedocs.adobe.com/acrobat_sdk/11/Acrobat11_HTMLHelp/JS_API_Acr oJS.89.1260.html
With no year information, it will give the current year. You will be able to get the day and the month as entered and use however you want. Note that Acrobat/Reader won't let you enter an invalid date (e.g., Feb 30) when the field is formatted as a date, so that's not a concern.