Interested in knowing how the forum would solve this. I have semi-regular data (example below is monthly but could be hourly or minute data +500k rows). I'm trying to put together a generic procedure that would work for all circumstances. What would be the best way to insert the missing rows? I'm thinking of generating all possible dates/times adding to the end and running a pivot table. That way I get rid of the time rounding, but the data also contains duplicates and other reasons why a simple average is not going to work which is why I'm inclined to sort and have a procedure step thru line by line and insert rows or flag duplicates.

INPUT DATA
Month,Year,Change,Rate
Aug,2013,-0.25,2.5
May,2013,-0.25,2.75
Dec,2012,-0.25,3
Oct,2012,-0.25,3.25
Jun,2012,-0.25,3.5
May,2012,-0.5,3.75

DESIRED OUTPUT
Month,Year,Change,Rate
May,2012,-0.5,3.75
Jun,2012,-0.25,3.5
Jul,2012,,
Aug,2012,,
Sep,2012,,
Oct,2012,-0.25,3.25
Nov,2012,,
Dec,2012,-0.25,3
Jan,2013,,
Feb,2013,,
Mar,2013,,
Apr,2013,,
May,2013,-0.25,2.75
Jun,2013,,
Jul,2013,,
Aug,2013,-0.25,2.5