How to determine the future date value in a 3-year rhythm

This sounds like a task for ECA Tamper. There are action plugins to convert between Unix timestamps and formatted dates. And there is also a requested new tamper plugin at ✨ Time offset tamper Needs work that lets you calculate time offsets (e.g. timestamp +3years). In combination of all those, you should be able to do what you described.

Thank you. I imagine the requested new tamper plugin could help here. I will try out.

It has turned out that the requirements in the related web project have changed and this calculation has now become obsolete. Nevertheless, I would like to see this problem solved out of interest in further training and also for the community.

In principle, the modelling approach would be as follows:
Add 3 years to the field_startdate and compare with current_date. If the result date is smaller (older) than the current_date, repeat this operation until the result date is in the future.
The eca_tamper plugin must be used to convert between Unix timestamps and formatted dates.

Unfortunately, I don't have the time this week to build the ECA-Moidel. The maintainers may therefore feel free to close this issue.