Invalid type for column time, must be of type timestamp or unix timestamp, got: string

Bom dia PessoALL,
Estou recebendo este alerta do grafana>
Invalid type for column time, must be of type timestamp or unix timestamp, got: string 2019.03

O que ocorre é que tenho uma SQL onde quero pegar somente mes/Ano
Olha ela aqui:

with Faturamento_Acumulado_por_mes as (

SELECT
case when c.Usage = 25 then a.WTSum + a.DiscSum else 0 end as ‘imposto’,
convert(nvarchar(7), a.DocDate,102) as time,

	sum(CASE 
			 WHEN (c.FreeChrgBP ='Y' AND SUBSTRING(c.itemcode,1,3) NOT IN ('PED','PRD','REV','MAP')) OR (a.Model = 56) THEN 0 -- se a operacao eh gratuita e nao eh produto acabado, revenda ou MP
			 ELSE c.LineTotal
		END) + sum(c.vatsum) as 'Total Max',


	'valor' as metric

		

	FROM OINV a
		 INNER JOIN INV12 b ON a.DocEntry = b.DocEntry
		 INNER JOIN INV1 c ON a.DocEntry = c.DocEntry

	WHERE 1=1
	
	 and c.USAGE not in ('14', '39')  -- utilização diferente de complemento de IPI, e revenda de imobilizado
	 and a.CANCELED not in ('Y','C')

group by convert(nvarchar(7), a.DocDate,102), case when c.Usage = 25 then a.WTSum + a.DiscSum else 0 end
 
	 
	)
	select
	t1.time as time,
	sum(t1.[Total Max] - t1.imposto) as value,
	t1.metric as metric

	 from Faturamento_Acumulado_por_mes t1

	 group by t1.time, t1.metric

	 order by t1.time desc

2019-03-25_11-00-18