In C#.NET, From

The .NET framework has a method that does exactly what you want:

int fourDigitYear = CultureInfo.CurrentCulture.Calendar.ToFourDigitYear(twoDigitYear)

Note : Beware of the date setting on each machine can be vastly different, since this CultureInfo is based on the machine setting. The result of this function can be unpredictable, e.g. on machines using Thai locale, BE is a preferred epoch, but it doesn’t make any sense with ToFourDigitYear. categories: dev

Workaround : You can set the current thread’s CultureInfo to something you’re sure of the CultureInfo behavior and then perform the call. After that return the current thread’s CultureInfo back to the old one.