Windowing Problem

[...] 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

Java Strategy

For parsing with the abbreviated year pattern ("y" or "yy"), SimpleDateFormat must interpret the abbreviated year relative to some century. It does this by adjusting dates to be within 80 years before and 20 years after the time the [...] instance is created.

Source: https://docs.oracle.com/javase/8/docs/api/java/text/SimpleDateFormat.html

Reference Year	Inputs
	'95'	'74'	'50'	'34'	'26'	'18'	'2'
2034	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
2015	`1995`	`1974`	`1950`	`2034`	`2026`	`2018`	`2002`
1997	`1995`	`1974`	`1950`	`1934`	`1926`	`1918`	`2002`
1986	`1995`	`1974`	`1950`	`1934`	`1926`	`1918`	`2002`

Posix Strategy

When a century is not otherwise specified, values in the range [69,99] shall refer to years 1969 to 1999 inclusive, and values in the range [00,68] shall refer to years 2000 to 2068 inclusive [...].

Source: https://pubs.opengroup.org/onlinepubs/009695399/functions/strptime.html

Reference Year	Inputs
	'95'	'74'	'50'	'34'	'26'	'18'	'2'
2034	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
2015	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
1997	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
1986	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`

Office Strategy

If you enter a date with a two-digit year in a text formatted cell or as a text argument in a function [...] Excel interprets the year as follows:

00 through 29: is interpreted as the years 2000 through 2029.
30 through 99: is interpreted as the years 1930 through 1999.

Source: https://docs.microsoft.com/en-us/office/troubleshoot/excel/two-digit-year-numbers

Reference Year	Inputs
	'95'	'74'	'50'	'34'	'26'	'18'	'2'
2034	`1995`	`1974`	`1950`	`1934`	`2026`	`2018`	`2002`
2015	`1995`	`1974`	`1950`	`1934`	`2026`	`2018`	`2002`
1997	`1995`	`1974`	`1950`	`1934`	`2026`	`2018`	`2002`
1986	`1995`	`1974`	`1950`	`1934`	`2026`	`2018`	`2002`

MySQL Strategy

Date values with 2-digit years are ambiguous because the century is unknown. Such values must be interpreted into 4-digit form because MySQL stores years internally using 4 digits.

For DATETIME, DATE, and TIMESTAMP types, MySQL interprets dates specified with ambiguous year values using these rules:

Year values in the range 00-69 become 2000-2069.
Year values in the range 70-99 become 1970-1999.

Source: https://dev.mysql.com/doc/refman/5.7/en/two-digit-years.html

Reference Year	Inputs
	'95'	'74'	'50'	'34'	'26'	'18'	'2'
2034	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
2015	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
1997	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`
1986	`1995`	`1974`	`2050`	`2034`	`2026`	`2018`	`2002`

IBM I Strategy

If a 2-digit year is moved to a 4-digit year, the century (1st 2 digits of the year) are chosen as follows:

If the 2-digit year is greater than or equal to 40, the century used is 1900. In other words, 19 becomes the first 2 digits of the 4-digit year.
If the 2-digit year is less than 40, the century used is 2000. In other words, 20 becomes the first 2 digits of the 4-digit year.

Source: https://www.ibm.com/docs/en/i/7.2?topic=mcdtdi-conversion-2-digit-years-4-digit-years-centuries

Reference Year	Inputs
	'95'	'74'	'50'	'34'	'26'	'18'	'2'
2034	`1995`	`1974`	`1950`	`2034`	`2026`	`2018`	`2002`
2015	`1995`	`1974`	`1950`	`2034`	`2026`	`2018`	`2002`
1997	`1995`	`1974`	`1950`	`2034`	`2026`	`2018`	`2002`
1986	`1995`	`1974`	`1950`	`2034`	`2026`	`2018`	`2002`

My Strategy

2-digit dates are ambiguous on the century. It is interpreted accordingly to the provided reference year as follows:

For values between 0 and the reference decade, it's the reference's century
For values greater than the current decade, it's the century before the reference's century

Source: EMANUEL, Lautaro Martin

Reference Year	Inputs
	'95'	'74'	'50'	'34'	'26'	'18'	'2'
2034	`1995`	`1974`	`1950`	`2034`	`2026`	`2018`	`2002`
2015	`1995`	`1974`	`1950`	`1934`	`1926`	`1918`	`2002`
1997	`1995`	`1974`	`1950`	`1934`	`1926`	`1918`	`1902`
1986	`1895`	`1974`	`1950`	`1934`	`1926`	`1918`	`1902`

The code for all these strategies can be found here

Other strategies:

emlautarom1/Windowing.md