[EpiData-list] How to analyse for outcomes at specific periods during follow ups

Dear ALL

I have a relational (Epidata entry) data base which I have data for over one year follow up of patients. I would like to analyse in Epidata analysis and check the Blood pressure, blood sugar readings......at every six months follow up for each patient since they were enrolled.

How do I get to select six month, twelve month....visits for each patient form their date of enrollment. ( in my database I have var: entrydate at registration and visitdate on each visit)

Thank you

Walter