Which makes more sense: Coding a website membership system based on a design pattern of renewing 1 year to the day of initial registration? Or coding one that renews everyone on June 30th regardless of when they first registered?

I would say it all depends.

Is there any legal basis for a fixed day, is it a payed membership?

For example, if the site regards memberships that require or involves access to some other service that is locked to a fixed date renewal or is it arbitrary.

@Voxera The client favors a fixed date renewal based on prior manual signup processes. Now that we are building an automatic process where people self-serve thru the website, the client still wants to use the single renewal date AND still manually update the database through exporting to Excel and then re importing. To me that throws red flags because arbitrary changes spell doom for data integrity in a rules-based system. If people self-serve, they can sign up for membership whenever is most convenient for them. That is a design pattern people have grown accustomed to. But if June 30 is the only signup date, why even have a web-based membership registration if you only allow it to be used once per year? Users will get frustrated and revenue will suffer. IMHO.

@stackodev I see, have seen it enough times when customers are moving from a manual system to our online based.

Our advantage is that we have a ready system with a thousand customers.

That gives us some weight when we tell them that there is a better way proven by existing customers, many that has been able to grow much faster with our system than before.