Значение проверки существует в предыдущем месяце в SQL

I have two table XYZ and ABC

create table #abc(id int, value int, dates date)
create table #xyz(id int, value int, dates date)

insert into #xyz values(1,10,'2017/02/01')
Insert into #xyz values(2,10,'2017/05/01')

Insert into #abc values(1,10,'2017/01/01')
Insert into #abc values(1,20,'2017/02/01')
Insert into #abc values(2,4,'2017/01/01')
Insert into #abc values(2,5,'2017/04/01')

XYZ
ID	Value	Dates
1	10	2017/02/01
2	10	2017/05/01

ABC			
ID	Values	Dates	ExpectedResult
1	10	2017/01/01	20
1	20	2017/02/01	0
2	4	2017/01/01	0
2	5	2017/04/01	15

i want to check if table ABC having (Month - 1 ) value comparing with table XYZ.
Uniqueness of table is identified by ID and Date  .
For eg-> Table XYZ  id-1 is having date 2017/02/01 i.e February data and my ABC table ID-1 is having 2017/01/01 i.e January data  then XYZ value + ABC value i.e 20 is the expected result.

Could you please tell me how do we compare month in this scenario.