Kierkegaard noticed something every quantitative researcher eventually learns the hard way: looking backward makes chaos look neat and predictable. In a Jupyter notebook, you already have the entire historical CSV sitting in memory. If a rolling average, scaler, or target label accidentally peeks even one day ahead (t + 1), your backtest looks effortless.
Real markets don't hand you tomorrow's row. Treating strict timestamp cutoffs as a non-negotiable rule — where the model at time t is completely blind to anything after t — is the difference between honest research and fooling yourself.