Every core banking modernisation pitch starts with the same slide: legacy system on the left, modern platform on the right, arrow in between. What that slide leaves out is the eighteen months in the middle, where both systems are live, both have to agree, and the business can’t stop moving money while you work.
The part that actually takes the time
Rewriting the core logic is rarely the bottleneck. Reconciling two systems of record during a phased cutover — and building enough confidence in that reconciliation to expand scope — is where most of the calendar time goes. Budget for it accordingly.