This change happened when all four digits of the year got a new value by changing from 1999 to 2000. Nearly all living people have always spoken the actual year as "nineteenhundred...". As a newborn child immediately is in its first year of life, our chronology starts with a 1st year instad of a year 0.
From one moment to another, we started to say "twothousand...". If we talk about the millennium, we use a well defined word that only makes sense if we relate it to our era. However, the lack of a year 0 was disturbing for the astronomers in their need for a continuously extendable time axis, so they invented (in contrast to our common chronology) the astronomical chronology.
the 19th birthday (literally: 19th anniversary of birth) or say that someone is 19 years old.
Rarely we talk about someone being within the 20th year of his life.
We celebrated the change, but let's stay linguistically correct: We celebrated the change of the first digit. Counting of the years "after the birth of Christ" (A. Before that, there was (besides others) the varronic era which started counting from the foundation of Rome. From the links below, you can see that the astronomers are very well at home within the Gregorian Calendar and have come to the right conclusions.
Because all people using our chronology have celebrated the millennium on January 1 of a year, we need not quarrel about the day. Cardinal counting gives an amount of elements (1 element, 2 elements, 3 elements, ...), whereas the ordinal counting enumerates and thereby names each of the elements (the 1st, the 2nd, the 3rd, ... That sounds very theoretical, but in everyday life we unconciously use cardinal numbers (c.n.) and ordinal numbers (o.n.): For measurement of time, e.g.: During the 90th minute (o.n.) of a soccer match, the stop-watch shows 89: ... For age informations: The 20th year (o.n.) of someone's life begins with his 19th birthday (literally: 19th anniversary of birth, o.n.) and lasts until one day before his 20th birthday.The ordinal counting of the days is still reflected in our language.But for the years, the accentuation of the ordinal counting (e.g.It should be the same with the numbers of the years. on 1/1/2000 our era should have been 2000 years old and not only 1999.When talking about the age of people, we mostly speak about e.g.Many centuries before, definitions were totally different.Thus, when speaking historically of the year 1, we must not take January 1 as beginning of the year.All the following is intended to deal with such objections. There are good reasons to start certain countings with 1 instead of beginning from 0. If a counting has the effect to give sort of a name to a counted element, you avoid the number 0.The word millennium stands for a big change in the counting of years. At first sight, that could be interpreted as an error, e.g. Instead of cardinal counting, you use ordinal counting which is - within its rules - mathematically correct (see 3b).Thus, its big 2000th birthday was after the end of the 2000th year of its life, that was on Annotation: We must not stretch this imagination too wide.Today's definition of our chronology includes that the New Year's Day always is on January 1.