RWDateTime has the ability to hold sentinel values that are not necessarily dates, but are convenient to use as comparison values within applications. For example, a “future” sentinel provides a value that represents a date larger than the largest RWDateTime. RWDateTime currently provides four internal, pre-defined sentinels and can accommodate up to 128 user-defined sentinels. The RWDateTime sentinels are described in the following sections.

Table 3 summarizes the responses of RWDateTime operations when applied to various sentinels. The sections that follow Table 3 provide more information about the sentinels.

The Essential Tools Module allows for RWDateTime instances with an “invalid” state that can be compared to other RWDateTime instances and can be persisted. The default RWDateTime constructor constructs an invalid instance, and other construction techniques are available, including the RWDateTime::invalidSentinel constant. You can pass this constant to the RWDateTime(rwint64) constructor to create instances of the minimum and maximum RWDateTime as needed, or you can use it to compare directly with the millisecond count returned by RWDateTime::milliSeconds().

No other operations work, other than persisting the invalid object and using it in comparisons of equality and inequality. Arithmetic manipulation of an invalid RWDateTime results in an invalid RWDateTime. Attempts to use the part extraction functions (day(), month(), extract(), etc.) result in an RWInternalErr being thrown by RWTHROW. The asString() family of functions always return the string "#INVALID#" when called with an invalid RWDateTime.

The following examples show several ways to construct an RWDateTime with an invalid state:

Sometimes it is convenient to have an RWDateTime that is valid yet holds no specific value. The Essential Tools Module includes the “null” state sentinel for this purpose. You can construct a null RWDateTime or use the constant RWDateTime::nullSentinel in construction by passing the constant RWDateTime::nullSentinel to the RWDateTime(rwint64) constructor. You can also use the constant in direct comparisons with the millisecond count returned by RWDateTime::milliSeconds().

Comparison operations for equality and inequality do work, with an RWDateTime in a “null” state always being less than a valid RWDateTime. Arithmetic manipulation of a null RWDateTime results in an invalid RWDateTime. Attempts to use any part extraction functions (day(), month(), extract(), etc.) result in an RWInternalErr being thrown by RWTHROW. The asString() family of functions always return the string NULL when called for a null RWDateTime.

The following examples show how to construct an RWDateTime with a null state, and demonstrate what happens when it is used in an arithmetic operation:

When sorting and searching for RWDateTime objects it is useful to have sentinels that represent “smaller than the smallest” RWDateTime and “larger than the largest” RWDateTime. The “past” and “future” sentinels are special kinds of invalid sentinels.

RWDateTime includes two global static constants to determine the minimum and maximum valid RWDateTime objects: RWDateTime::minDateTime and RWDateTime::maxDateTime. You can pass these constants to the RWDateTime(rwint64) constructor to create instances of the minimum and maximum RWDateTime as needed, or you can use them to compare directly with the millisecond count returned by RWDateTime::milliSeconds().

The past and future sentinels can be used in any relational operation, and every valid RWDateTime falls strictly between the past and future sentinels. Arithmetic manipulation of past or future sentinels does not change their values, nor cause any error. Attempts to use any part extraction functions (day(), month(), extract(), etc.) result in an RWInternalErr being thrown by RWTHROW. The asString() family of functions always return the string "#INVALID#" for past and future sentinels.

Here are some examples of constructing and using past and future sentinels:

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: