Only a few days left of this year. After that we will be entering into a new decade: twenties. Or are we?
Some people thing the decade doesn't start until the year one e.g. this decade started in 2011 and will last until the end of year 2020. Mathematically that makes sense. The way we count from one and up to ten. There also wasn't year zero in Anno Domini era, so strictly speaking this interpretation is also true if we just count all the ten years along the way. Otherwise counting decades wouldn't add up to a current year.
On the other hand it makes more sense to consider the years with the same leading digits grouped together, be it decades, centuries or millenniums. Would sound weird to consider 1990 belonging to the eighties for example. Ignoring the missing year zero this way we could consider the current year being the one already belonging to next decade when the year number would tell the count of elapsed years. So we are currently already living the 2020th year, the last one of this decade.
As a computer engineer I also support the x0-x9 decades instead of the x1-(x+1)0 definition. In this field we have been used to start counting from zero.