Hello I am trying to calculate the rolling average of the past 36 months (periods) and when I run the code using rangestat the new variable results in all missing values. I am not sure how to fix this and output the correct calculation.

Here is an example of my sample:

Code:
input double date long _ID float country_pdsi
4.733856e+11 9  1.5002956
 4.76064e+11 9  1.7449586
4.784832e+11 9  1.6589595
4.811616e+11 9  1.4847733
4.837536e+11 9  1.0025214
 4.86432e+11 9   .8517559
 4.89024e+11 9   .7509255
4.917024e+11 9   .7988663

format %tc date
This is my code to calculate the 36 month moving average.

Code:
xtset _ID date
rangestat (mean) country_pdsi, interval(date -36 -1) by(_ID)