Lots of systems store events that happen at moments in time. But what if the timestamps can have more than one meaning. Tom the co-founder of Finbourne explains how they use bi-temporal data and event sourcing to build a consistent view of portfolio data where they can look back at any point in time and find out the two truths. What did system see at that time and what did the world see. Find out the difference between "effective at" and "as at" and learn what happens when you need to make corrections to the timeline.