BA-08 · LESSON 11

Block time, MedianTimePast, and future limits

KEY DEFINITIONS

Terms used in this lesson

Proof-of-work target: The maximum header-hash value accepted for a block; lowering it makes valid hashes harder to find.

Validate a candidate timestamp against the median of the previous eleven blocks and the receiving node's two-hour future-time limit.

55 min Difficulty 3/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · block time validationA candidate block time must pass two contextual boundsSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered validation pipeline

01
11 ANCESTORSSelected parent branch
The parent and ten earlier ancestors supply exactly eleven timestamp integers.
02
SORTAscending timestamps
Sorting avoids treating arrival order or one extreme value as the time reference.
03
MEDIANItem 6 = 1700003010
MedianTimePast is the middle item of the sorted eleven values.
04
CANDIDATEMedian + 1 second
The lower-bound comparison is strict, so equality fails.
05
NODE RESULTCheck future limit, then accept
The same node also rejects more than two hours beyond adjusted network time.

Rule shown: Header time must be strictly greater than the previous eleven-block median and no more than two hours beyond adjusted network time when received.

STAGE 1 OF 511 ANCESTORS: Selected parent branch

The parent and ten earlier ancestors supply exactly eleven timestamp integers.

Governing relationship: Header time must be strictly greater than the previous eleven-block median and no more than two hours beyond adjusted network time when received. Complete path: 1. 11 ANCESTORS, Selected parent branch. The parent and ten earlier ancestors supply exactly eleven timestamp integers. 2. SORT, Ascending timestamps. Sorting avoids treating arrival order or one extreme value as the time reference. 3. MEDIAN, Item 6 = 1700003010. MedianTimePast is the middle item of the sorted eleven values. 4. CANDIDATE, Median + 1 second. The lower-bound comparison is strict, so equality fails. 5. NODE RESULT, Check future limit, then accept. The same node also rejects more than two hours beyond adjusted network time.

1. State transitionWhat exists, what changes, and what results

BEFORE

The sorted preceding times have a median of 1,700,003,010 and the node's adjusted time is 1,700,006,100.

MECHANISM

Follow the candidate's selected parent branch and read the timestamps of that parent plus its ten ancestors. Times on a different branch do not enter this candidate's MedianT…

AFTER

Block time is miner-proposed but bounded by chain history and the receiving node's adjusted clock.

Text equivalent: Starting from The sorted preceding times have a median of 1,700,003,010 and the node's adjusted time is 1,700,006,100. The mechanism begins when Follow the candidate's selected parent branch and read the timestamps of that parent plus its ten ancestors. Times on a different branch do not enter this candidate's MedianTimePast. The resulting lesson is Block time is miner-proposed but bounded by chain history and the receiving node's adjusted clock.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Collect the previous eleven times

Follow the candidate's selected parent branch and read the timestamps of that parent plus its ten ancestors. Times on a different branch do not enter this candidate's MedianTimePast.

Complete path: 1. Collect the previous eleven times: Follow the candidate's selected parent branch and read the timestamps of that parent plus its ten ancestors. Times on a different branch do not enter this candidate's MedianTimePast. 2. Sort and choose the median: Sort all eleven integer timestamps and choose item six. The median resists one extreme timestamp more effectively than trusting the previous block alone. 3. Apply the strict lower bound: The candidate header time must be greater than MedianTimePast. Equality fails. This rule gives locktime evaluation a monotonic chain-derived time reference. 4. Apply the receiving-node upper bound: When a node receives the header, its time must be no more than two hours beyond adjusted network time. This is a node-relative admission boundary rather than a field committed elsewhere in the chain. 5. Recompute after a reorg: A branch change produces a different set of eleven ancestors and may therefore produce a different MedianTimePast. Context belongs to the selected parent chain.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The node derives MedianTimePast from the candidate's parent branch and compares the header integer with both contextual boundaries.

CHANGED CONDITION

Using average time, only the prior block, eleven blocks from the wrong branch, greater-than-or-equal, or the local wall clock alone applies a different rule.

Important distinction: Every accepted Bitcoin block must be exactly ten minutes after the previous block. Ten minutes is the long-run target interval. Individual block times vary, and timestamp validity uses MedianTimePast plus a future-time limit rather than a ten-minute schedule.

Text equivalent: The accepted path is enforced by The node derives MedianTimePast from the candidate's parent branch and compares the header integer with both contextual boundaries. If the required condition changes, Using average time, only the prior block, eleven blocks from the wrong branch, greater-than-or-equal, or the local wall clock alone applies a different rule.

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Learn the idea

A block header carries one unsigned 32-bit Unix timestamp, but that number is not accepted simply because a miner wrote it. Contextual validation requires the candidate time to be strictly greater than MedianTimePast, the median of the preceding eleven block timestamps. A node also rejects a newly received block whose time is more than two hours ahead of its adjusted network time.

GUIDED EXPLANATION1/5 ideas inspected

Collect the previous eleven times

Follow the candidate's selected parent branch and read the timestamps of that parent plus its ten ancestors. Times on a different branch do not enter this candidate's MedianTimePast.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Every accepted Bitcoin block must be exactly ten minutes after the previous block.”

Ten minutes is the long-run target interval. Individual block times vary, and timestamp validity uses MedianTimePast plus a future-time limit rather than a ten-minute schedule.

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