create difference column of time series in MySQL
Hope this helps. It's SQL Server code, but you should be able to perform the same join on your table with the date_add function.

DECLARE @TABLE TABLE ([Date] Datetime, [Value] int);

INSERT INTO @TABLE VALUES ('2015-Sep-01',11);
INSERT INTO @TABLE VALUES ('2015-Sep-02',13);
INSERT INTO @TABLE VALUES ('2015-Sep-03',15);
INSERT INTO @TABLE VALUES ('2015-Sep-04',20);
INSERT INTO @TABLE VALUES ('2015-Sep-05',68);

SELECT 
    Main.Date, 
    Main.Value - Previous.Value as [Difference]  
FROM @TABLE Main
LEFT OUTER JOIN @TABLE Previous 
    ON Previous.[Date] = DATEADD(DAY,-1, Main.Date);