This would be easier to answer if we could see the workbook but my guess is that you just need to create another series where the incremental values are the same magnitude but scaled over the longer period. For instance if your data is spread over the 31 days of January then derive the daily increments but scale them by 31/38.

Either that where you adjust the values or you scale the dates so that the same 31 cells of dates are equally distributed between 38 days