[isqad88]

<span id="date_time_start_t"></span>
    <button id="f_btn1">...</button>
    <script type="text/javascript">//<![CDATA[
      (function(){
        Calendar.setup({
        inputField : "date_time_start_t",
        trigger    : "f_btn1",
        onSelect   : function() { this.hide() },
        showTime   : 24,
        dateFormat : "%d-%m-%Y %H:%M"
      });
     }).call(this);
    //]]></script>

А если так?