[...] you may [...] be stuck with data containing two-digit year fields. You have tried to get it expanded to four digits at the source, but it simply cannot be done.
In this case, you need to adopt a technique called "windowing". Windowing means taking the two-digit year and applying common sense to determine the century that it belongs in.
Obviously, windowing cannot be used for data that might span a period greater than 100 years. For instance, the birth years of people in the general population. There have always been a number of people living beyond the age of 100, and as health care improves that number can only increase. If you code a birth year as "96", that could be either 1996, which would indicate a two-year-old, or 1896, which could indicate a 102-year-old. For a short time after the year 2000, there will be people alive who were born in three different centuries!
Source: https://www.uic.edu/depts/accc/software/isodates/fixing.html