"December 21st, 2017"
The Chinese writing describing a numerically written "year" now often always uses the number zero that looks like the English capital letter O used to denote the Chinese word "ling". This "ling" means zero or nothing. So the word for the year 2O18 should be said as "er ling shi ba". This word makes more sense than just "er (blank) shi ba". Trust me, this would be way better to understand the sound of saying a year in "putonghua". Thirty years ago the writing of the chinese year was totally different than recently while computers used now make it easier.
"Putonghua" is the answer. It only started circa 1949 and was officially used in 1980. To speak and write the year now is said as individual single numbers: So to say 2018 = 2, 0, 1, 8 = er ling yi ba. However to write it with putonghua they just use numerical numbers and the capital letter O = zero = ling. They don't use the number 0 to say or mean ling. The use of this was by putonghua developing the use of pinyin largely on international use of English and 5 other European languages used on computers. Duolingo unfortunately hasn't corrected this and you don't say ling in the first place. Chinese language has involved and is involving in time just like any other languages. Hope this helps. You should check about the "Putonghua" on the internet. It may help you understand how it has change the use of Mandarin.