Timing cost refers to the return penalty incurred by being out of the market during its best-performing days. The mathematics is stark: historically, a disproportionate share of long-term equity returns has been concentrated in a relatively small number of large single-day gains. An investor who is on the sidelines — even for a handful of those days — experiences a compounding deficit that is difficult to overcome.
The argument is often used to dismiss market timing entirely. However, the honest version of the analysis cuts both ways: the same small number of concentrated days logic applies to the worst days too. Missing the worst days improves returns comparably. A timing strategy that successfully avoids the worst days while capturing most of the best ones would beat buy-and-hold — the empirical question is whether any real-world strategy achieves that with sufficient consistency.
Timing cost is the reason the burden of proof falls on any timing strategy: it must generate enough reduction in drawdown (or improvement in risk-adjusted return) to outweigh the certain cost of the missed-best-days drag. The Timing Cost Simulator on this site lets you model the specific trade-off numerically, rather than accepting the headline statistic uncritically.