Try something like

<xp:dateTimeHelper dojoType="dijit.form.DateTextBox"
                                    id="dateTimeHelper">
   <xp:this.dojoAttributes>
            <xp:dojoAttribute name="constraints">
                 <xp:this.value><![CDATA[#{javascript:return minmaxDatum()}]]></xp:this.value>
            </xp:dojoAttribute>
    </xp:this.dojoAttributes>
</xp:dateTimeHelper>

the function for minmaxDatum returns a mindate (in my case the start of a year) and a maxdate (here today)

the function pad is called to return the month and day with 2 digits (required for the constraints to work) 

function minmaxDatum() {

     function pad(n){return n<10 ? '0'+n : n}

     var jaar = sessionScope.AppSettings.get("boekJaar");
    var jaari = parseInt(jaar,10);
    var date1 = @Today();
    var year = @Year(date1);
    var month = @Month(date1);
    var day = @Day(date1);
   
    var vandaag = new Date(@Today());   
    var min = "min:'" + jaari + "-01-01'";
    var max = "max:'" + year + "-" + pad(month) + "-" + pad(day) + "'";
    return "{" + min + "," + max + "}";
}  

Thank you very much indeed !