Datatypes incompatible when concatenating variable

I'm using the following stored procedure to transfer data from a staging table into a production table in SQL Server 2014.

 DECLARE @p NVARCHAR(250), @s NVARCHAR(250), @c NVARCHAR(20), @cid VARCHAR(10), @sql NVARCHAR(MAX), @d DATE
 SET @p = @prod_tbl
 SET @s = @stg_tbl
 SET @c = @customer
 SET @d = GETDATE()

 SELECT @cid = [CustomerID]  FROM [dbo].[Customers] WHERE [CustomerName] LIKE '%' + @c +'%'
 
 SET @sql = 'INSERT INTO ' + @p + ' (AggregateName,TimeIndex,TotalSpace,UsedSpace,UsedPercent,DaysToFull,DailyGrowth,ControllerName,CustomerID)
                SELECT SUBSTRING(Aggregate,1,25),
                '+@d+',
                SUBSTRING(Total,1,15),
                SUBSTRING(Used,1,15),
                CONVERT(decimal(10,3),UsedPerc,3),
                CONVERT(int,DaysToFull),
                CONVERT(decimal(10,3),DailyGrowthRatePerc,3),
                SUBSTRING(Controller/Cluster,1,25),
                '''+@cid+''' FROM ' + @s

 EXEC sp_executesql @sql

@p, @s, and @c are parameters passed to the procedure but on the third line of the INSERT INTO statement I'm using the @d variable trying to insert the current date into a datetime column in the production table which doesn't exist in the staging table.

Whatever I'm doing wrong is giving me the following:

The data types nvarchar and date are incompatible in the add operator.

Here's the production table I'm inserting into:

CREATE TABLE [dbo].[AggregateCap](
        [AggrCapID] [int] IDENTITY(1,1) NOT NULL,
        [AggregateName] [varchar](25) NOT NULL,
        [TimeIndex] [datetime] NOT NULL,
        [TotalSpace] [varchar](15) NOT NULL,
        [UsedSpace] [varchar](15) NOT NULL,
        [UsedPercent] [decimal](10, 2) NOT NULL,
        [DaysToFull] [int] NOT NULL,
        [DailyGrowth] [decimal](10, 2) NOT NULL,
        [ControllerName] [varchar](25) NOT NULL,
        [CustomerID] [varchar](10) NOT NULL,
 CONSTRAINT [PK_AggregateCap] PRIMARY KEY CLUSTERED 
(
        [AggrCapID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

and I'm transferring from a staging table like so:

CREATE TABLE [dbo].[staging-table](
        [ObjectID] [varchar](100) NULL,
        [Aggregate] [varchar](100) NULL,
        [Controller/Cluster] [varchar](100) NULL,
        [Total] [varchar](100) NULL,
        [Used] [varchar](100) NULL,
        [UsedPerc] [varchar](100) NULL,
        [Committed] [varchar](100) NULL,
        [CommittedPerc] [varchar](100) NULL,
        [DailyGrowthRatePerc] [varchar](100) NULL,
        [DaysToFull] [varchar](100) NULL
) ON [PRIMARY]

Any ideas as to what I'm doing wrong? Thanks in advance

Adam