RWDateTime also provides for the use of 128 user-defined sentinels numbered 0 through 127. Each of these sentinels behaves like the null sentinel for part extraction functions, arithmetic manipulation, and relational operations. The asString() family of functions return "#>num<#" where num is the user sentinel number. The static method RWDateTime::userSentinel(int) is available for constructing user sentinels. These can be used to define different kind of dates, such as “refused”, for cases where a user has declined to enter data in a date field.

Here is an example of constructing and using user-defined sentinels: