ex 27/01/2014, 27/02/2014, 27/03/2014 etc
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.