Corrections are to the median block times, which were generated by subtracting
CBlockIndex::nMedianTimeSpan / 2 from the block height and then multiplying by
the target spacing. GetMedianTimePast() takes an array sorted by std::sort() and
returns element CBlockIndex::nMedianTimeSpan / 2, meaning that if
CBlockIndex::nMedianTimeSpan is odd (which it is), there is an out-by-one error
in the subtraction.
