I would like to calculate an estimate of the monthly population with population data sorted by region, gender, age group, and year.
I calculated the estimate by subtracting the previous year's data from the following year's data and adding the monthly value divided by 12.
For example, since the population as of January 1998 was 23424, and 1999 was 25832 (25832-23424)/12 was added to 23424 and estimated the value for February 1998.
What do I write to create time (year and month) variables and calculate the estimate, as shown in the figure from the left to the right in the attached figure?




Thank you in advance for your help. Array