Dear R users, I have a panel data set (in MS excel)  on prices across countries and time country    time                 price   1         "23/11/08"            2 1   "28/12/08"                   3 1    "25/01/09"                   4 1   "22/02/09"                   5 1    "29/03/09"                  6 1  "26/04/09"                   32 1  "24/05/09"                   23 1  "28/06/09"                   32 2   "26/10/08"                45 2  "23/11/08"                 46 2  "21/12/08"               90 2  "18/01/09"                54 2  "15/02/09"                 65 2   "16/03/09"               77 2  "12/04/09"                    7 2   "10/05/09"                   6 As you can see, 1)the start and end date of the time series for countries 1 and 2 are different. For example, for country 1 the time series begins on "23/11/08" while for country 2 the time series begins on "26-10-2008”. 2)My data on prices are available every 28 days (or equivalently every 4 weeks). So, each observation is a 4-week average. But in some cases I have jumps (35 days or 29 days instead of 28 days).  Examples are: "28/12/08" to "28/12/08" , from 22/02/09" to "29/03/09", etc My goal is to have a unified sequence of dates across countries. Otherwise I will not be able to do further data/econometric analysis, Unless you have different suggestion, I want  to take what I have and calculate monthly average prices. To be more specific, I want to change the frequency of the time series (probably using the EXPAND procedure?) Put differently, I want to interpolate the 4-week average prices to monthly average prices. The problem is also the jumps where I have 5 weeks in some cases and I want to find the monthly average of it. I am a new user of SAS which means that a suggestion, although welcome, will not be very helpful. I would be grateful if you could provide an exact code adjusted to the above example If parts  of my question are not very understandable, please feel free to ask me thank you ... View more