user41513: All miners at any time must have a consensus on the "difficulty target" to be able to achieve the correct nonce (as answer of proof-of-work).
Assume that the target has been updated (it means that 2016 blocks has been discovered) now, the question is "who" calculates this new target such that every miner achieves the same target that is needed to solve proof-of-work of the next block? 
Is it true that it's not possible for an attacker to manipulate the target value to trick the other miners such that they achieve a wrong nonce for proof-of-work? 
In other words, how can every miner know that the target value has been changed? 
--> Is it by the block header of the last block in the Blockchain? 
If so, is this approach accountable to achieve a unique target?
Assume 2016 blocks have been discovered and the target must be updated using the following equation:
time = (difficulty x 2^32) / hashing power of the network
Is it right? It's unclear to me how to determine the value for hashing power of the network.
P.S. Assume also an adversary solves the proof-of-work and then generates a new block but forges the block's times-stamp to trick the network at the time of updating target. Since the target will be updated using the blocks' time-stamp.

user30732: To address the remark in the PS, i.e. 
P.S. Assume also an adversary solves the proof-of-work and then generates a new block but forges the block's times-stamp to trick the network at the time of updating target. Since the target will be updated using the blocks' time-stamp.
That is indeed possible, it's called "timejacking" or the time traveller attack, you can find out more about it here: 
http://culubas.blogspot.de/2011/05/timejacking-bitcoin_802.html
or here: 
bitcoins from the future

user24974: Because the difficulty calculation is deterministic, everyone can calculate the difficulty independently.  Given that they have come to consensus on the blockchain (which blocks in which order), they should all calculate the same value.  In this way, the protocol never relies on trusting anyone about what the correct difficulty is.
Also, keep in mind that it's not just miners that would have to be fooled into accepting a block with insufficient work.  Full nodes also verify blocks as they receive them, and will reject blocks that do not have nonces that meet their own calculated difficulty requirement.  Since full nodes play an important role in relaying blocks, this should not be overlooked.
If you'd like to look at how the difficulty calculation is performed, it's in the code at: https://github.com/bitcoin/bitcoin/blob/master/src/pow.cpp

user3041: Assuming the miner as an up-to-date version of the blockchain, he can compute the difficulty from the data about the previous blocks. An explanation of how the difficulty is computed can be found in this other answer. 
Given a specific chain, indeed, the computation of the difficulty always gives the same result. Every miner may compute this new difficulty by himself and the result will be the same since the computation is based on the 2016 blocks slot. There is no need to know the hashrate of the network, it is enough to know how much time it was needed to generate the 2016 blocks by looking at their timestamps. 
Additionally, block headers also encode the target threshold through the nBits field. The reason can be found in this answer.
PS: In order for a block to be considered valid, its timestamp must also be valid, that is, it must be:

greater than the median timestamp of previous 11 blocks, and less than
  the network-adjusted time + 2 hours.

So the effects of the "attack" are very constrained.

41513: <p>All miners at any time must have a consensus on the "<em>difficulty target</em>" to be able to achieve the correct nonce (as answer of proof-of-work).</p>

<p>Assume that the target has been updated (it means that 2016 blocks has been discovered) now, the question is "<strong>who</strong>" calculates this new target such that every miner achieves the same target that is needed to solve proof-of-work of the next block? </p>

<p>Is it true that it's not possible for an attacker to manipulate the target value to trick the other miners such that they achieve a wrong nonce for proof-of-work? </p>

<p>In other words, how can every miner know that the <strong>target value has been changed</strong>? </p>

<p>--> Is it by the block header of the last block in the Blockchain? </p>

<p>If so, is this approach accountable to achieve a unique target?</p>

<p>Assume 2016 blocks have been discovered and the target must be updated using the following equation:</p>

<p>time = (difficulty x 2^32) / hashing power of the network</p>

<p>Is it right? It's unclear to me how to determine the value for <strong>hashing power of the network</strong>.</p>

<p>P.S. Assume also an adversary solves the proof-of-work and then generates a new block but forges the block's times-stamp to trick the network at the time of updating target. Since the target will be updated using the blocks' time-stamp.</p>
 30732: <p>To address the remark in the PS, i.e. </p>

<p><code>P.S. Assume also an adversary solves the proof-of-work and then generates a new block but forges the block's times-stamp to trick the network at the time of updating target. Since the target will be updated using the blocks' time-stamp.</code></p>

<p>That is indeed possible, it's called "<em>timejacking</em>" or the time traveller attack, you can find out more about it here: </p>

<p><a href="http://culubas.blogspot.de/2011/05/timejacking-bitcoin_802.html" rel="nofollow noreferrer">http://culubas.blogspot.de/2011/05/timejacking-bitcoin_802.html</a></p>

<p>or here: </p>

<p><a href="https://bitcoin.stackexchange.com/questions/4271/bitcoins-from-the-future">bitcoins from the future</a></p>
 24974: <p>Because the difficulty calculation is deterministic, everyone can calculate the difficulty independently.  Given that they have come to consensus on the blockchain (which blocks in which order), they should all calculate the same value.  In this way, the protocol never relies on trusting anyone about what the correct difficulty is.</p>

<p>Also, keep in mind that it's not just miners that would have to be fooled into accepting a block with insufficient work.  Full nodes also verify blocks as they receive them, and will reject blocks that do not have nonces that meet their own calculated difficulty requirement.  Since full nodes play an important role in relaying blocks, this should not be overlooked.</p>

<p>If you'd like to look at how the difficulty calculation is performed, it's in the code at: <a href="https://github.com/bitcoin/bitcoin/blob/master/src/pow.cpp" rel="nofollow">https://github.com/bitcoin/bitcoin/blob/master/src/pow.cpp</a></p>
 3041: <p>Assuming the miner as an up-to-date version of the blockchain, he can compute the difficulty from the data about the previous blocks. An explanation of how the difficulty is computed can be found in <a href="https://bitcoin.stackexchange.com/questions/5838/how-is-difficulty-calculated/5840#5840">this other answer</a>. </p>

<p>Given a specific chain, indeed, the computation of the difficulty always gives the same result. Every miner may compute this new difficulty by himself and the result will be the same since the computation is based on the 2016 blocks slot. There is no need to know the hashrate of the network, it is enough to know how much time it was needed to generate the 2016 blocks by looking at their timestamps. </p>

<p>Additionally, block headers also encode the target threshold through the <a href="https://bitcoin.org/en/developer-reference#block-headers" rel="nofollow noreferrer">nBits field</a>. The reason can be found in <a href="https://bitcoin.stackexchange.com/questions/34155/is-it-necessary-for-blocks-to-record-the-nbits-difficulty-parameter">this answer</a>.</p>

<p>PS: In order for a block to be considered valid, its timestamp must also be valid, that is, it must be:</p>

<blockquote>
  <p>greater than the median timestamp of previous 11 blocks, and less than
  the network-adjusted time + 2 hours.</p>
</blockquote>

<p>So the effects of the "attack" are very constrained.</p>
