<p>Transactions don't have a timestamp. <em>Blocks</em> have a timestamp. The difference is important, because the block timestamp on some of the blocks changes the difficulty.</p>

<blockquote>
  <p>Why is the maximum difference two hours?</p>
</blockquote>

<p>It's not particularly important to have very accurate timestamps. Timestamps have two uses:</p>

<ul>
<li><a href="https://bitcoin.stackexchange.com/questions/5838/how-is-difficulty-calculated">Difficulty retargeting</a></li>
<li>Calculating progress of synchronization. If the blockchain started in 2009, and the last block you saw was made in 2012, then you're about halfway done.</li>
</ul>

<blockquote>
  <p>Or, if this is about time zones and summer time, shouldn't they be using UTC time?</p>
</blockquote>

<p>They do. Of course, not all systems are set up correctly. For example, imagine a system that doesn't use NTP. The user doesn't know how to set up DST correctly, so they move their computer's clock forward/backward an hour every spring/fall.</p>

CC BY-SA 3.0
2014-09-20T23:40:57.440Z
2014-09-20T23:40:57.440Z
2017-04-13T12:47:33.660Z
Nick ODell
<p>As far as I understand, Bitcoin transactions include a timestamp, but the only requirement is that they don't differ for more than two hours.</p>

<p>But why two hours? For computers that are constantly synced via NTP, two hours seem like a very big difference. I would expect that a difference of 10 minutes should be enough.</p>

<p>Or, if this is about time zones and summer time, shouldn't they be using UTC time?</p>

<protocol><timestamp><block-validity>
Why is two hours the maximum acceptable offset for Bitcoin blocks?
Arturo Torres Sánchez
# Why is two hours the maximum acceptable offset for Bitcoin blocks?

(Asked by: username_0 on 2014-09-20)

As far as I understand, Bitcoin transactions include a timestamp, but the only requirement is that they don't differ for more than two hours.

But why two hours? For computers that are constantly synced via NTP, two hours seem like a very big difference. I would expect that a difference of 10 minutes should be enough.

Or, if this is about time zones and summer time, shouldn't they be using UTC time?

**Tags:** protocol, timestamp, block-validity


---

## 1 answers

### Answer by username_1 on 2014-09-20. Score: 7 | Accepted answer

Transactions don't have a timestamp. *Blocks* have a timestamp. The difference is important, because the block timestamp on some of the blocks changes the difficulty.

> Why is the maximum difference two hours?

It's not particularly important to have very accurate timestamps. Timestamps have two uses:

* [Difficulty retargeting](https://bitcoin.stackexchange.com/questions/5838/how-is-difficulty-calculated)
* Calculating progress of synchronization. If the blockchain started in 2009, and the last block you saw was made in 2012, then you're about halfway done.

> Or, if this is about time zones and summer time, shouldn't they be using UTC time?

They do. Of course, not all systems are set up correctly. For example, imagine a system that doesn't use NTP. The user doesn't know how to set up DST correctly, so they move their computer's clock forward/backward an hour every spring/fall.


---

