i want to keep the day constant but increase the month by 1 in every cell how

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Enter two dates into two cells, like 27/01/2014 and 27/02/2014, then select both cells and pull them down at the bottom right handle.

Or enter just one date 27/01/2014 and select a cell range including that cell at the top and invoke menu Sheet -> Fill Cells... -> Fill Series..., Series Type Date, Time Unit Month, Increment 1.