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 1/1/99 to 1/1/01, 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 1/1/30 or higher, it will be treated internally as 1/1/1930 or higher. All lower two-digit years apply to the 20xx century. So, for example, 1/1/20 is converted into 1/1/2020.