I was wondering whether it is correct to include baseline year as additional covariate in a Cox regression model when using data from an open cohort. I will try to give an example to make this clearer:
Data: open cohort of individuals followed up from 1990 to 2016
Baseline: year of diagnosis of hypertension
Exposure: lifestyle intervention in the year following the diagnosis
Outcome: fatal cardiovascular event
In this case each individual would enter the cohort the year of diagnosis of hypertension and that year would be the baseline year. as individuals are censored at different time the model would account for the differences in the follow-up. As that is the baseline year, all covariates (i.e. age, sex, ethnicity, BMI..) registered in that year would be included. As this is meant to be a non-time varying model, covariates would not change over time and no interaction term would be fitted with time. However, the time period is quite extended and there are changes in guidelines in how to treat the condition over time, therefore, I would be keen to include in the model year of diagnosis (baseline year) as additional covariate (categorical variable with first year as ref). To me this would better account for the possible changes in management over time. However, I could not find a good reference that would answer my question about whether this is a correct approach. Any advice?