MTP (Median Time Past) — How Bitcoin Tracks Network Time

Median Time Past (MTP) is the median of the previous 11 block headers' timestamps. It's used as the "current network time" reference for time-based consensus rules in Bitcoin, replacing the more easily-manipulated per-block timestamp.

The problem MTP solves: miners have limited but real discretion over block timestamps. The protocol allows a block's timestamp to be off by up to a couple of hours from real time, within bounds set by previous blocks. A single miner could push their timestamp forward to manipulate time-based logic - like making time-locked outputs spendable earlier than the depositor intended.

How MTP fixes it: by taking the median of the previous 11 blocks' timestamps, the network gets a value that:

• Resists single-miner manipulation. One miner setting a wildly skewed timestamp doesn't move the median significantly.
• Stays close to real time. The median of 11 honest miners' timestamps approximates real time well, within a small bias.
• Increases monotonically. As new blocks arrive, MTP can only move forward, providing a stable monotonic clock.

Where MTP is used:

• BIP-113 locktimes. Time-based nLockTime and CLTV evaluations use MTP as the reference, not the current block's timestamp.
• BIP-68 relative locktimes and CSV.
• Various other time-sensitive consensus rules.

For most users, MTP is invisible. For protocol designers building time-locked constructions, it's the precise notion of "time" Bitcoin actually uses internally. Treating block timestamps as raw real-time gives wrong answers; treating MTP as real-time gives essentially correct ones.

The 11-block window is a sweet spot: large enough to resist manipulation, small enough to stay close to real time. Average lag from real time is around 60 minutes (half the window).