function showNextMonth( formulario ){
		var mes = parseInt(formulario.mes.value);
		var anyo = parseInt( formulario.anyo.value );
		
		if ( mes < 12 ) mes = mes + 1;
		else {
			anyo = anyo + 1;
			mes = 1;
		}		 
		formulario.mes.value = mes;
		formulario.anyo.value = anyo;
		formulario.submit();
	}

	function showPreviousMonth( formulario ){
		var mes = parseInt(formulario.mes.value);
		var anyo = parseInt( formulario.anyo.value );
		
		if ( mes > 1 ) mes = mes - 1;
		else {
			anyo = anyo - 1;
			mes = 12;
		}		 
		formulario.mes.value = mes;
		formulario.anyo.value = anyo;
		formulario.submit();
	}	

	function showNextYear( formulario ){
		var anyo = parseInt( formulario.anyo.value );
		formulario.anyo.value = anyo + 1;
		formulario.submit();
	}

	function showPreviousYear( formulario ){
		var anyo = parseInt( formulario.anyo.value );
		if ( anyo > 0 ){
			formulario.anyo.value = anyo - 1;
			formulario.submit();
		}
	}