It is up to the evaluator, developer, and test staff to determine what that is for each product. It may not be sufficient to simply observe the product across each of the identified critical dates. Date horizons determine the product's time interval of interest. A forecasting system that predicts staffing requirements for the next six months has a leading date horizon of six months. Similarly, a program that calculates productivity over the past month has a trailing date horizon of one month.
Some systems have only a single date horizon, either leading or trailing, and some have both.
The figure above illustrates the concept of date horizons. If the date horizons are so long that the test setup cannot be tied up to permit the continuous flow of leading date horizon, time now, and trailing date horizon to cross the critical date, the scenarios need to be broken into pieces. In such cases, care must be taken to ensure that the test data from the leading date horizon, through time now, through trailing date horizon, is consistent so that a true picture of product operation can be obtained. A Word of Caution Testers and evaluators must be careful when executing test scenarios that involve advanced dates. Simply setting the system clock forward may cause unanticipated effects.