Adding Timestamp to Exported Filename in SQL Server Management Studio
This gives date and time both and make it windows compatible filename as well

DECLARE @dt AS VARCHAR(30)
SELECT @dt = REPLACE( CONVERT(VARCHAR(26),getdate(),120),':','-')
EXEC xp_cmdshell 'bcp "SELECT * FROM dbo.tablename" queryout "C:\filename-' + @dt +'" -T -c -t, '