This approach buys you only a few years, depending on the application. You still must eventually change all date programs and databases.
You cannot use the century window forever because a 2-digit year can only be unique in a given 100-year period. Over time you will need more than 100 years for your data window—in fact, many companies need more than 100 years now.
The reason that the century window buys you more time is that you know in any given section of ILE COBOL code whether you are trying to figure out if a date is old (the date is in the past) or if a due date has not yet been reached (the date is in the future). You can then use this knowledge to determine how to set your century window.
There are limitations, though. For example, the century window cannot solve the problem of trying to figure out how long a customer has been with your company, if the time-span is greater than 100 years and you only have 2-digit years in your dates. Another example is sorting. All of the records that you want to sort by date must have 4-digit year dates. For these problems and others, you need to use ACCEPT statements, Intrinsic Functions, or ILE date services which return a 4-digit year.