//Get the last date of the month. If in Feb 2012, lastDate is 29.
var lastDate = Ext.Date.getDaysInMonth(new Date());
//15th
var middleDate = 15;
//Construct regular expression
var disabledArray=[];
var today = Ext.Date.format(new Date(), 'm/d/Y');
var dateReg = /(\d{2}\/)\d{2}(\/\d{4})/;
disabledArray.push(today.replace(dateReg, '$1' + middleDate + '$2'));
disabledArray.push(today.replace(dateReg, '$1' + lastDate + '$2'));
//Something like "^(?!02/15/2012|02/29/2012).*$" including the two days allowed.
var disabledReg = '^(?!' + disabledArray.join('|') + ').*$';
//Apply the regular expression to date field
var dateField = new Ext.form.field.Date({
format: 'm/d/Y',
disabledDates: [disabledReg]
});