Each block has a timestamp. Each 2016 blocks difficulty is adjusted so that next 2016 blocks are produced closer to 10 minute interval between the blocks. Nodes do not tolerate timestamps 2 hours in the future relative to their own timestamps: https://github.com/bitcoin/bitcoin/blob/master/src/main.cpp#...
Since difficulty is adjusted based on a simple median, if the hashrate is always growing (like for the past 8 months), average time between blocks is always below the target 10 minutes (it was around 7-8 minutes on average).
Since difficulty is adjusted based on a simple median, if the hashrate is always growing (like for the past 8 months), average time between blocks is always below the target 10 minutes (it was around 7-8 minutes on average).