If you are printing the date (dateTime) in a Text element, set Date pattern (in Properties->Specific tab for the Text element) property.
As an alternate, you can use a JavaScript function to change the date format. These functions can be readily found in .js files under %rpe_home%\utils\scripts, if you are using RPE 2.1 or later. Here is a sample function:
//function to change date format to "MMM dd, yyyy"
function changeDateFormat(date)
{
    try
    {
        var currDtFmt = new java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    var curDt = currDtFmt.parse(date);
    var expDtFmt = new java.text.SimpleDateFormat("MMM dd, yyyy");
    var dateString = expDtFmt.format(curDt).toString()
    _sessionLogger.info("##### data string: " + dateString);
    return dateString;
    }
    catch(err)
    {
        java.lang.System.out.print( err);
    return date;
    }
}