Really need help! With the expert guidance of a statistician, I merged (using the append command) a baseline data set with three registry-based data sets (using the append command). Now, my goal is to only keep the registry-based HbA1c test dates (spanning from 01jan2008 to 31dec2020) that were performed 1 year (364.25 days) after each participant's baseline HbA1c value, which was collected in each original study participant from 01oct2008 to 01apr2010.

Before merging, the original study’s data set was labeled as baseline = 1 and had the following variables in as the columns in long format: ID_number, the HbA1c test date (status_date), the HbA1c value (HbA1c_mmolmol), the year the HbA1c test was done (y_status_date), the absolute # of days between each participant's birthday and the date of their HbA1c test (new_diff_days), baseline (labeled as 1 for the original study) and some string variables. Before merging / appending the data sets, the registry-based data was labeled as baseline = 1 and it has all of the same variable names and labels as the original study without any string variables. My question = What code should I use to only keep the registry-based (baseline == 0) HbA1c values and test dates that were done 1 year (364.25 days) after each participant's baseline study's HbA1c test date?


[CODE]
* Example generated by -dataex-. For more info, type help dataex
clear
input double(ID_number status_date HbA1c_mmolmol) float(y_status_date new_diff_days baseline)

000000000 17875 61 2008 343 0
111111111 17979 61 2009 81 1
222222222 18071 66 2009 173 0
333333333 18281 64. 2017 18 0
444444444 18788 62 2019 160 0
555555555 19025 59 2009 32 1

The above dataex example is a dummy data set. It is not my real data.