function show_calendar(str_target, str_datetime) {
        var arr_months = ["Enero", "Febrero", "Marzo", "Abril", "Mayo", "Junio","Julio", "Agosto", "Septiembre", "Octubre", "Noviembre", "Diciembre"];
        var week_days = ["Do", "Lu", "Ma", "Mi", "Ju", "Vi", "Sa"];
        var n_weekstart = 1; // dia se la semana que empieza (normalmente 0 ó 1)

        var dt_datetime = (str_datetime == null || str_datetime =="" ?  new Date() : str2dt(str_datetime));
        var dt_prev_month = new Date(dt_datetime);
        dt_prev_month.setMonth(dt_datetime.getMonth()-1);
        var dt_next_month = new Date(dt_datetime);
        dt_next_month.setMonth(dt_datetime.getMonth()+1);
        var dt_firstday = new Date(dt_datetime);
        dt_firstday.setDate(1);
        dt_firstday.setDate(1-(7+dt_firstday.getDay()-n_weekstart)%7);
        var dt_lastday = new Date(dt_next_month);
        dt_lastday.setDate(0);

        // visualiza la cabecera del calendario
        var str_buffer = new String (
                "<html>\n"+
                "<head>\n"+
                "        <title>Historial</title>\n"+
                "</head>\n"+
                "<body bgcolor=\"#4682B4\" topmargin=\"0\" leftmargin=\"0\">\n"+
                "<table class=\"clsOTable\" cellspacing=\"0\" border=\"0\" width=\"100%\">\n"+
                "<tr><td bgcolor=\"#4682B4\">\n"+
                "<table cellspacing=\"1\" cellpadding=\"3\" border=\"0\" width=\"100%\">\n"+
                "<tr>\n        <td bgcolor=\"#4682B4\"><a href=\"javascript:window.opener.show_calendar('"+
                str_target+"', '"+ dt2dtstr(dt_prev_month)+"'+document.cal.time.value);\">"+
                "<img src=\"imagenes/ant.gif\" width=\"16\" height=\"16\" border=\"0\""+
                " alt=\"Mes anterior\"></a></td>\n"+
                "        <td bgcolor=\"#4682B4\" colspan=\"5\" align=\"center\">"+
                "<font color=\"white\" face=\"tahoma, verdana\" size=\"2\">"
                +arr_months[dt_datetime.getMonth()]+" "+dt_datetime.getFullYear()+"</font></td>\n"+
                "        <td bgcolor=\"#4682B4\" align=\"right\"><a href=\"javascript:window.opener.show_calendar('"
                +str_target+"', '"+dt2dtstr(dt_next_month)+"'+document.cal.time.value);\">"+
                "<img src=\"imagenes/sig.gif\" width=\"16\" height=\"16\" border=\"0\""+
                " alt=\"Siguiente mes\"></a></td>\n</tr>\n"
        );

        var dt_current_day = new Date(dt_firstday);
        // visualiza los días de la semana
        str_buffer += "<tr>\n";
        for (var n=0; n<7; n++)
                str_buffer += "        <td bgcolor=\"#87CEFA\" align=\"center\">"+
                "<font color=\"white\" face=\"tahoma, verdana\" size=\"2\">"+
                week_days[(n_weekstart+n)%7]+"</font></td>\n";
        // visualiza la tabla del calendario
        str_buffer += "</tr>\n";
        while (dt_current_day.getMonth() == dt_datetime.getMonth() ||
                dt_current_day.getMonth() == dt_firstday.getMonth()) {
                // print row heder
                str_buffer += "<tr>\n";
                for (var n_current_wday=0; n_current_wday<7; n_current_wday++) {
                                if (dt_current_day.getDate() == dt_datetime.getDate() &&
                                        dt_current_day.getMonth() == dt_datetime.getMonth())
                                        // fecha actual
                                        str_buffer += "        <td bgcolor=\"#FFB6C1\" align=\"right\">";
                                else if (dt_current_day.getDay() == 0 || dt_current_day.getDay() == 6)
                                        // weekend days
                                        str_buffer += "        <td bgcolor=\"#DBEAF5\" align=\"right\">";
                                else
                                        // print working days of current month
                                        str_buffer += "        <td bgcolor=\"white\" align=\"right\">";

                                if (dt_current_day.getMonth() == dt_datetime.getMonth())
                                        // print days of current month
                                        str_buffer += "<a href=\"javascript:window.opener."+str_target+".value='"+dt2dtstr(dt_current_day)+"'+document.cal.time.value; window.close();\">"+
                                        "<font color=\"black\" face=\"tahoma, verdana\" size=\"2\">";
                                else
                                        // print days of other months
                                        str_buffer += "<a href=\"javascript:window.opener."+str_target+
                                        ".value='"+dt2dtstr(dt_current_day)+"'+document.cal.time.value; window.close();\">"+
                                        "<font color=\"gray\" face=\"tahoma, verdana\" size=\"2\">";
                                str_buffer += dt_current_day.getDate()+"</font></a></td>\n";
                                dt_current_day.setDate(dt_current_day.getDate()+1);
                }
                // print row footer
                str_buffer += "</tr>\n";
        }
        // print calendar footer
        str_buffer +=
                "<form name=\"cal\">\n<tr><td colspan=\"7\" bgcolor=\"#87CEFA\">"+
                "<font color=\"White\" face=\"tahoma, verdana\" size=\"2\">"+
                "<input type=\"hidden\" name=\"time\" value=\""+dt2tmstr(dt_datetime)+
                "\" size=\"8\" maxlength=\"8\"></font></td></tr>\n</form>\n" +
                "</table>\n" +
                "</tr>\n</td>\n</table>\n" +
                "</body>\n" +
                "</html>\n";

       if(screen.width == 1024) { // Corro el calendario para ajustarlo al formulario
         var vWinCal = window.open("", "Calendar","width=200,height=188,status=no,resizable=no,top=410,left=600");
       } else if (screen.width == 1280) {
         var vWinCal = window.open("", "Calendar","width=200,height=188,status=no,resizable=no,top=410,left=730");
       } else {
         var vWinCal = window.open("", "Calendar","width=200,height=188,status=no,resizable=no,top=410,left=500");
       }
         vWinCal.opener = self;
         var calc_doc = vWinCal.document;
         calc_doc.write (str_buffer);
         calc_doc.close();
}
// datetime parsing and formatting routimes. modify them if you wish other datetime format
function str2dt (str_datetime) {
        var re_date = /^(\d+)\-(\d+)\-(\d+)\s+(\d+)\:(\d+)\:(\d+)$/;
        if (!re_date.exec(str_datetime))
                return alert("Formato de fecha incorrecto: "+ str_datetime);
        return (new Date (RegExp.$3, RegExp.$2-1, RegExp.$1, RegExp.$4, RegExp.$5, RegExp.$6));
}
function dt2dtstr (dt_datetime) {
        return (new String (
                        dt_datetime.getDate()+"-"+(dt_datetime.getMonth()+1)+"-"+dt_datetime.getFullYear()+" "));
}
function dt2tmstr (dt_datetime) {
        return (new String (
                        dt_datetime.getHours()+":"+dt_datetime.getMinutes()+":"+dt_datetime.getSeconds()));
}
