How do you generate "virtual" recurring transaction projections in a backend?

Hi everyone,

Stack: Node.js / Nuxt (Nitro) backend, TypeScript, PostgreSQL. DB schema is already settled, so I'm not looking for feedback on that — I need help with the backend implementation logic itself.

Context: personal finance app. A recurrences table stores rules (amount, frequency, due day, total installments if applicable, start date). Real transactions live in a separate transactions table with a nullable recurrence_id FK. Future occurrences are NOT pre-generated in the DB — they should be calculated on demand as "virtual" projections (e.g. "show me all transactions, real + projected, for July 2026"), and only turned into a real row when the date arrives or the user confirms it.

What I need help with, specifically:

1. How would you structure the function/service that takes a recurrence rule + a date range and returns the list of projected occurrences? Any concrete approach or pseudocode.
2. Would you reach for a library (date-fns, rrule.js, luxon) to generate the occurrence dates, or roll your own date math? If a library, which one and why.
3. How do you merge real transactions and virtual projections into a single list for the frontend without confusing the two (flags, separate response shape, IDs)?
4. Once a projection needs to become a real row (date arrives, or user confirms), what's a clean way to do that without duplicating logic between the "projection calculator" and the "materializer"?

Not looking for database/schema advice — just the backend implementation approach. Any pseudocode, patterns, or library recommendations are welcome. Thanks!