How to extract month on from the date field in SQL

See more:

SQL-Server-2014

I have a date field name as "InvoiceDate" in my table that have a date information like this "20140231", I want to get the month out of that date field so that I can group my record per quarter see the attached query that I tried, currently the date field is hard coded as you see on the query.
Can you please help how can I get the month on this record

"20140231", 

What I have tried:

This what I tried

SELECT TOP 1000 [Advise no]
      ,[Amount]
      ,[Date]
      ,[Bas Fund Description]
	  ,[Invoice Date]
         (MONTH, DATEDIFF(MONTH, 0, [Invoice Date]), 0) AS months
	   , CASE WHEN [Invoice Date] BETWEEN 20170401 AND 20170631
             THEN 'Q1' 
           WHEN [Invoice Date] BETWEEN 20170701 AND 20170931
             THEN 'Q2' 
           WHEN [Invoice Date] BETWEEN 20171001 AND 20171231
             THEN 'Q3'
           WHEN [Invoice Date] BETWEEN 20180101 AND 20180331 
              THEN 'Q4' 
           WHEN [Invoice Date] Is Null 
              THEN 'Null' 

        END AS QuarterPeriod 
      ,[Valuation Date]
      ,[qgs_geometry]
      ,[qgs_fid]
      ,[ProvinceName]
  FROM [DOD].[dbo].[View_DPW_PACE]