Median Time Past (MTP)

To prevent miners from manipulating the clock (for example, by setting a block's time far in the past to influence the difficulty adjustment), Bitcoin nodes use a calculation called Median Time Past (MTP).

1. The Calculation

The MTP is the median timestamp of the last 11 blocks.

• Why 11? 11 blocks (roughly 1 hour and 50 minutes) is a large enough window to smooth out any individual miner's clock errors or intentional manipulations.

• The Rule: A new block's timestamp must be greater than the Median Time Past of the previous 11 blocks.

2. BIP 113: Consensus Enforcement

Originally, MTP was used for difficulty adjustments, but not for validating transactions. In 2016, BIP 113 made MTP the standard for enforcing Timelocks (nLockTime).

• Old Rule: Use the block's actual timestamp.

• New Rule: Use the MTP of the previous 11 blocks.

This makes timelocks more predictable and prevents "time-warp" attacks where miners could potentially spend locked coins prematurely.

3. Monotonicity

The MTP rule ensures that the "consensus clock" is monotonic, meaning it only ever moves forward. Even if a miner's local clock is slightly slow, they cannot set a timestamp that is earlier than the average of the recent history.

4. Example Scenario

If the last 11 blocks have these timestamps (simplified): 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150

• The median value is 125.

• The next block (Block 12) must have a timestamp of 126 or higher, even if the miner's current local time is 124.

[!NOTE] MTP effectively puts a "floor" on the block time. It ensures that the blockchain history moves forward in a logical, verifiable sequence that cannot be rolled back by a single malicious actor.

Next, we will look at the "ceiling" of the block time: the 2-Hour Future Rule.