$filter_date_from = $this->state->get("filter.date_from_dateformat");
    $filter_Qdate_from = (!empty($filter_date_from)) ? $this->isValidDate($filter_date_from) : null;

    if ($filter_Qdate_from != null)
    {
        $query->where("a.date = '" . $db->escape($filter_Qdate_from) . "'");
    }

<field name="date_from_dateformat" label="COM_MERAKIAC_LIST_LBL_SESSION_DATE_FILTER_FROM" type="calendar" onchange="this.form.submit();" filter="safehtml" class="inputbox"/>

<script>
Calendar.setup({
    cont: "sample1",
    inputField: "filter_date_from_dateformat"
});
</script>

<div id="sample1">
<input type="text" title="" name="filter[date_from_dateformat]" id="filter_date_from_dateformat" value="" maxlength="45" class="inputbox hasTooltip" onchange="this.form.submit();" />
</div>