You can use the special field _creationDate
as creation date (see answer from Tommy). Based on this the following example query will work in the Notes client:
Field _creationDate > 01-01-2014 AND Field _creationDate < 01-03-2014
In order to get this query to work with your specific code do this:
var tmpArray = new Array("");
var cTerms = 0;
var dateFormatter = new java.text.SimpleDateFormat( "dd-MM-yyyy" );
if (sessionScope.searchDate1) && (sessionScope.searchDate2) {
tmpArray[cTerms++] = "Field _creationDate > " + dateFormatter.format(sessionScope.searchDate1) + " AND Field _creationDate < " + dateFormatter.format(sessionScope.searchDate2);
}