Basically what they did was the following. Each concrete block represents 10 years. They get placed when another decade ends/starts(not to sure). So the problem is the following: when the 1200 years are over they have only placed 119 blocks so the building will be finished in 1210 years. Its the same as with a fence. If you have e.g. 10 Elements of a fence you need 11 posts. The ones who designed this building however didnt count the fence posts(the concrete blocks which mark the end of a decade) and I stead counted the "fence elements" causing the whole thing to be off by 10 years.