This is a bug, but I was asked to report here after first reporting to Customer Support

After purchasing a new training plan, it is possible to automatically apply starting "next Monday".

However, for international users, the localised intended date (e.g. 05/01/2026, January 5th) is then interpreted as a US format (MM/DD/YYYY) date, and the plan is applied starting May 1st.

Note that no date is ever actually input or selected by the user, this mixup is all internal to TrainingPeaks