JavaScript function after clicking on clear button
function resetTime() {
    document.getElementById("to_time").valueAsDate = null;
}
PrimeFaces calendar tag
<p:calendar id="to_time"
            value="#{anomaliesList.toDate}"
            readonlyInput="true"
            maxdate="#{currentDate}"
            size="16"
            pattern="dd/MM/yyyy HH:mm" />
 
    