If the field is formatted as date ("dd.mm.yyy") you can add the following script as a Custom validation script:
var v = event.value;
var d = util.scand("dd.mm.yyyy", v);
var d1 = util.scand("dd.mm.yyyy", "30.06.2015");
if(d > d1){
event.value = "";
app.alert("You can only enter a date until the 30.06.2015.");
}