[runarberg]

In Icelandic the 1-based towards counting is used almost everywhere. People do indeed say: “The first decade of the 19th century” to refer to the 18-aughts, and the 90s is commonly referred to as “The tenth decade”. This is also done to age ranges, people in their 20s (or 21-30 more precisely) are said to be þrítugsaldur (in the thirty age). Even the hour is sometime counted towards (though this is more rare among young folks), “að ganga fimm” (or going 5) means 16:01-17:00.

Speaking for my self, this doesn’t become any more intuitive the more you use this, people constantly confuse decades and get insulted by age ranges (and freaked out when suddenly the clock is “going five”). People are actually starting to refer to the 90s as nían (the nine) and the 20-aughts as tían (the ten). Thought I don’t think it will stick. When I want to be unambiguous and non-confusing I usually add the -og-eitthvað (and something) as a suffix to a year ending with zero, so the 20th century becomes nítjánhundruð-og-eitthvað, the 1990s, nítíu-og-eitthvað and a person in their 20s (including 20) becomes tuttugu-og-eitthvað.