Dynamic Backup File in SQL

insert into TableMaster values(1,cast(getdate() as char(10)))

select * from TableMaster

USE MASTER;
DECLARE  @FileName NVARCHAR(1024)
SET @FileName = 'C:\backups\fullTest_' + REPLACE(REPLACE(CONVERT(VARCHAR(100),GETDATE(),111),'/','_'), ' ','_') + '.BAK'

SET @FileName = REPLACE(@FileName,'__','_')

BACKUP DATABASE demostart TO  DISK = @FileName WITH NOFORMAT,
INIT

--Differential backup
USE MASTER;
DECLARE  @FileName NVARCHAR(1024)
SET @FileName = 'C:\backups\fullTest_' + REPLACE(REPLACE(CONVERT(VARCHAR(100),GETDATE(),121),'-','_'), ' ','_') + '.BAK'

SET @FileName = REPLACE(@FileName,'__','_')
--print @FileNAme
BACKUP DATABASE demostart TO  DISK = @FileName WITH NOFORMAT,
INIT