Applying the Sliding Window Technique

In this section:

When to Supply Settings for DEFCENT and YRTHRESH
Date Validation

To apply the sliding window technique correctly, you need to understand the difference between a date format (formerly called a smart date) and a legacy date:

A date format refers to an internally stored integer that represents the number of days between a real date value and a base date (either December 31, 1900, for dates with YMD or YYMD format; or January 1901, for dates with YM, YYM, YQ, or YYQ format). A Master File does not specify a data type or length for a date format; instead, it specifies display options such as D (day), M (month), Y (2-digit year), or YY (4-digit year). For example, MDYY in the USAGE (also known as FORMAT) attribute of a Master File is a date format. A real date value such as March 5, 1999, displays as 03/05/1999, and is internally stored as the offset from December 31, 1900.
A legacy date refers to an integer, packed decimal, double precision, floating point, or alphanumeric format with date edit options, such as I6YMD, A6MDY, I8YYMD, or A8MDYY. For example, A6MDY is a 6-byte alphanumeric string; the suffix MDY indicates how Information Builders will return the data in the field. The sample value 030599 displays as 03/05/99.

For details on date fields, see the Describing Data manual.

When to Supply Settings for DEFCENT and YRTHRESH

The rest of this topic refers simply to DEFCENT when either DEFCENT or FDEFCENT applies, and to YRTHRESH when either YRTHRESH or FYRTHRESH applies.

Supply settings for DEFCENT and YRTHRESH in the following cases:

When you issue a DEFINE or COMPUTE command to convert a legacy date without century digits to a date format with century digits (for example, to convert the format I6YMD to YYMD). With DEFINE and COMPUTE, DEFCENT and YRTHRESH do not work directly on legacy dates; for example, you cannot use them to convert the legacy date format I6YMD to the legacy date format I8YYMD.
When a DEFINE command, COMPUTE command, or Dialogue Manager -SET command calls a function, supplied by Information Builders, that uses legacy dates, the input date does not contain century digits.
On input, the function will use the window defined for an 16 legacy date field (with edit options). The output format may be 18 (again, with edit options), which includes a 4-digit year.

When data is entered or changed in a date format field in a FOCUS data source, or a SQL date is entered or changed in a Relational Database Management System (RDBMS), and the input date does not contain century digits.
For example, you can use the sliding window technique in applications that use FIXFORM or CRTFORM with MODFIY.

When a data source is read, and the ACTUAL attribute in the Master File is non-date specific (for example, A6, I6, or P6), without century digits, and the FORMAT or USAGE attribute specifies a date format. This case does not apply to FOCUS data sources.

Follow these rules when implementing the sliding window technique:

Specify values for both DEFCENT and YRTHRESH to ensure consistent coding and accurate results, except when YRTHRESH has a negative value. In that case, specify a value for YRTHRESH only; do not code a value for DEFCENT.
Do not use DEFCENT and YRTHRESH with ON TABLE SET.

Finally, keep in mind that the sliding window technique does not change the way existing data is stored. Rather, it accurately interprets data during application processing.

Applying the Sliding Window Technique

When to Supply Settings for DEFCENT and YRTHRESH

Date Validation