The Sliding Window Technique

In this section:

Defining a Sliding Window
Creating a Dynamic Window Based on the Current Year

With the sliding window technique, you do not need to change stored data from a 2-digit year format to a 4-digit year format in order to determine the century. Instead, you can continue storing 2-digit years and expand them when accessed.

The sliding window technique recognizes that the earliest and latest values for a single date field in most business applications are within 100 years of one another. For example, a human resources application typically contains a field for the birth date of each active employee. The difference in the birth date (or age) of the oldest active employee and the youngest active employee is not likely to be more than 100.

The technique is implemented as follows:

You define the start of a 100-year sliding window by supplying two values: one for the default century (DEFCENT) and one for the year threshold (YRTHRESH). For example, a value of 19 for the century, combined with a value of 60 for the threshold, creates a window that starts in 1960 and ends in 2059.
The threshold provides a way to assign a value to the century of a 2-digit year:
- A year greater than or equal to the threshold assumes the value of the default century (DEFCENT). Using the sample value 19 for the default century and 60 for the threshold, a 2-digit year of 70 is interpreted as 1970 (70 is greater than 60).
- A year less than the threshold assumes the value of the default century plus 1 (DEFCENT + 1). Using the same sample values (19 and 60), a 2-digit year of 50 is interpreted as 2050 (50 is less than 60), and a 2-digit year of 00 is interpreted as 2000 (00 is also less than 60).
The conversion rule for this example is illustrated as follows:

Any 2-digit year is assumed to fall within the window. You must handle dates that fall outside the defined window by coding.

Each file or each date field used in an application can have its own conversion rule, which provides the flexibility required by most applications.

Defining a Sliding Window

You can define a sliding window in several ways, depending on the specific requirements of your application:

Globally. The SET DEFCENT and SET YRTHRESH commands define a window on a global level.
On a file level. The FDEFCENT and FYRTHRESH attributes in a Master File define a window on a file level, allowing the correct interpretation of date fields from multiple files that span different time periods.
On a field level. The DEFCENT and YRTHRESH attributes in a Master File define a window on a field level, allowing the correct interpretation of date fields, within a single file, that span different time periods.
For a virtual field. The DEFCENT and YRTHRESH parameters on a DEFINE command, in either a request or a Master File, define a window for a virtual field.
For a calculated value. The DEFCENT and YRTHRESH parameters on a COMPUTE command define a window for a calculated value.

If you define more than one window using any of the preceding methods, the precedence is as follows:

DEFCENT and YRTHRESH on a DEFINE or COMPUTE command.
DEFCENT and YRTHRESH field-level attributes in a Master File.
FDEFCENT and FYRTHRESH file-level attributes in a Master File.
SET DEFCENT and SET YRTHRESH on a global level; if you do not specify values, the defaults are used (DEFCENT = 19, YRTHRESH = 0).

Creating a Dynamic Window Based on the Current Year

An optional feature of the sliding window technique enables you to create a dynamic window, defining the start of a 100-year span based on the current year. The start year and threshold for the window automatically change at the beginning of each new year.

If an application requires that a window's start year change when a new year begins, use of this feature avoids the necessity of manually re-coding it.

To implement this feature, YRTHRESH or FYRTHRESH is offset from the current year, or given a negative value.

For example, if the current year is 1999 and YRTHRESH is set to -38, a window from 1961 to 2060 is created. The start year 1961 is derived by subtracting 38 (the value of YRTHRESH) from 1999 (the current year). To interpret dates that fall within this window, the threshold 61 is used.

At the beginning of the year 2000, a new window from 1962 to 2061 is automatically created; for dates that fall within this window, the threshold 62 is used. In the year 2001, the window becomes 1963 to 2062, and the threshold is 63, and so on.

With each new year, the start year for the window is incremented by one.

When using this feature, do not code a value for DEFCENT or FDEFCENT, since the feature is designed to automatically calculate the value for the default century. Be aware of the following:

If you do code a value for DEFCENT on the field level in a Master File, or for FDEFCENT on the file level in a Master File, the feature will not work as intended. The value for the century, which is automatically calculated by YRTHRESH by design, will be reset to the value you code for DEFCENT or FDEFCENT.
If you code a value for DEFCENT anywhere other than the field level in a Master File (for example, on the global level), and YRTHRESH is negative, the coded value will be ignored. The default century will be automatically calculated as designed.