The year in a date entry is often entered as two digits. Internally, the year is managed by LibreOffice as four digits, so that in the calculation of the difference from 01/01/99 to 01/01/01, the result will correctly be two years.
Under LibreOffice - PreferencesTools - Options - LibreOffice - General you can define the century that is used when you enter a year with only two digits. The default is 1930 to 2029.
This means that if you enter a date of 01/01/30 or higher, it will be converted to the internal date 1930-01-01 or higher. All lower two-digit years are converted to the 20xx century. So, for example, 01/01/20 is converted into 2020-01-01.