Hi there!

I am a rather novice Stata user doing an MPH. I am using Stata 15 IC on Windows 10; my data is in long format.

I am trying to classify visits to a clinic using the date of the visit. I am only interested in annual visits, and the way the visits were captured was for all visits, including interim. Visit 1 for all participants is the enrollment visit, then for some participants visit 2 is 12 months after visit 1 (example 294 below) whilst others visit 2 is 6 months after visit 1 (example 295 below). I have tried to look for an answer on forums and videos, but can't find quite the right approach. I thought to calculate the months since visit 1 and then create categories based on that, but can't figure out how to do that either. Please see example of data below. Any assistance would be greatly appreciated.

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