When I started learning English in junior school I was told that I had to pronounce the year 1997 nineteen ninety-seven and the year 2007 two thousand seven.

I've always followed the rule and pronounced the current year two thousand something, while refering to the last century with nineteen-something.

However I have, on several occasions, heard people talk about future years - e.g. 2050 in this tune's intro - and pronounce it twenty something.

This made me think it was ok to say, for instance, twenty seventeen when talking about the current year and I thought about it no more. Lately, in English class, I was talking to my teacher about a trip I did in 2012 and as he asked when, I answered: "That was in twenty twelve". And well by the look on his face I imediately corrected myself - "Oops, I mean two thousand twelve!" - and he was happy. But I did not get the occasion of asking him about the rule.

When will we stop refering to the current year as two thousand something and start saying twenty something?

Two thousand seventeen is already long and laborious to say, but in ten years? Two thousand twenty-seven is even worse. By then will we be "allowed" to say twenty twenty-seven? If not, when? Why not now? And what about the year 4456, do I have to say four thousand four hundreds fifty six or fourty-four fifty-six?

To make short, what is the rule here? I have no clue.

Thanks in advance!

P.S.: I also have no clue on how to hyphenate numbers but that's another story.