Costs and benefits are usually incurred over time, and the timing of costs and benefits is different for different interventions.

For example, the costs of a physiotherapy programme after hip replacement are incurred over a fixed, short period of time (say, six weeks). Similarly, the benefits accrue in a short time.
In contrast, the costs of statin intervention for secondary prevention of cardiovascular events are incurred over a long time. Similarly, the benefits of the treatment accrue over many years.

It's important that we are able to take this into account when we do economic analysis.