Expansion: Expand all 2 digit dates to four digits. The most straightforward technique.
Compression: Squeeze 4 digit dates into 2 digit slots and provide a coding/decoding algorithm.
Windowing: Split the 2 digit date range (00 - 99) into 2 sections at some date “XY”. Interpret all date prior to “XY” as being in the 21st century and all date from (and including) “XY” s being in the 20th century. The date “XY” is known as the pivot date. A variant of this technique allows “XY” to change over time as data storage requirements change.
Encapsulation: Freezing the system date and provide a translation algorithm at the I/O. This would only be used if there are not enough resources to change the entire code.
Setback: Set the system date back 28 years (2000 becomes 1972). This year was chosen because it mirrors 2000 in days, dates, and leap year. This is a form of encapsulation and would also require a translation algorithm at the I/O.