Timeline normalisation and uncertainty

# Timeline normalisation and uncertainty

Normalise offset timestamps to UTC, correct known clock error and decide when evidence does or does not establish strict temporal order.

This is an original, fictional, offline exercise for an introductory security learner. It is independent practice, not copied coursework or a claim of professional experience. Completion and competence remain unassessed.

## Before you start

Allow **30-45 minutes**: 5 minutes reading, 10 inspecting and predicting, 10 checking and explaining, and 5-20 for a changed or independent case. You need Python 3.10+ (standard library only), a terminal and a text editor. Comfort opening files, reading a small table and basic evidence notes is enough; programming, administrator rights, a Linux installation and cloud accounts are unnecessary. Previous introductory network or Wireshark practice is useful but not required.

> **Vocabulary:** UTC offset: the displayed timezone difference from UTC. Clock skew: device clock error relative to a reference. Uncertainty bound: the allowed interval around an estimate. Causation: one event producing another, which timing alone cannot prove.

## Evidence and exact rules

The case JSON is the single authoritative input. Fixtures are guided.json, changed.json and challenge.json. All data is synthetic; artifacts is empty to avoid competing editable copies. Edit only a prepared run copy. The analyser is a pure calculation and never executes evidence text.

data.events provides ID, timestamp with seconds and explicit Z or numeric offset, signed clock_offset_seconds, uncertainty_seconds and description. Offset is bounded to +/-86400 and uncertainty to 0-3600 seconds. data.pairs names two different known event IDs and a unique comparison ID. Naive dates and impossible timestamps are rejected.

Convert the recorded timestamp to UTC, then subtract clock_offset_seconds: positive means a fast device clock. Construct a closed interval centre +/- uncertainty. The left event is definitely-before only when its upper bound is strictly less than the right lower bound; definitely-after uses the reverse strict comparison. Every overlap, touching endpoint or identical timestamp gives order-uncertain. Ordering is temporal only and never evidence of causation by itself.

## Run the exercise

Open a terminal in this lab folder. Use your Python command (`python`, or `py -3` if that is how Python is installed). Commands below use `python` consistently.

```text
python lab.py cases
python lab.py prepare --scenario guided --out run/attempt-01
python lab.py show --case run/attempt-01/case.json
```

Read the evidence. In run/attempt-01/answers.json, fill each decision with an exact label from the rules; add an explanation and evidence references such as record IDs or configuration line numbers. In notes.txt, separate observation, interpretation, uncertainty, next check and help used. Save your prediction before opening the answer.

```text
python lab.py analyse --case run/attempt-01/case.json
python lab.py check --case run/attempt-01/case.json --answers run/attempt-01/answers.json
python lab.py prepare --scenario changed --out run/attempt-02
python lab.py prepare --scenario challenge --out run/attempt-03
python -m unittest -v test_lab.py
python lab.py verify
```

For changed/challenge use the corresponding attempt folder in show/analyse/check. prepare refuses an existing output folder. --out is relative to this lab and must be below run/; explicit input paths are relative to your terminal directory. analyse/show default to the guided fixture if --case is omitted. Exit 0 means command success or decision match; 1 means differences/incomplete written fields; 2 means invalid input or a filesystem problem. The checker verifies decision fields and the presence of writing, not the quality of reasoning. verify checks shipped examples, not learner mastery.

## Deliverable and limits

Keep case.json, answers.json and notes.txt together in the ignored run folder. Deliver a short handover: the question, your conclusion, two precise evidence references, one alternative explanation or limitation, and the next evidence you would request. State whether you used the worked example and whether you completed a fresh case unaided. Do not describe an automated match as assessment or professional experience.

A positive clock offset means the device clock is fast and is subtracted after UTC normalisation. The supplied uncertainty is a bound, not a probability. The model assumes supplied offset and uncertainty estimates are valid; collection delay, timestamp granularity and omitted events need separate investigation.

Use WORKED-EXAMPLE.md after making a first prediction, and DRILLS.md for progressively less support. Do not use these rules as an operational security verdict or modify a real host as part of this exercise.

## Primary references

The following documentation supports the underlying concepts; the tiny schema and explicit teaching rules above are specific to this lab. Links were checked during authoring on 23 October 2026. Running the lab needs no internet connection.

- [NIST Guide to Computer Security Log Management](https://csrc.nist.gov/pubs/sp/800/92/final)

# Worked example: Timeline normalisation and uncertainty

Try the guided case before reading this page. Use `python lab.py prepare --scenario guided --out run/worked-01` if you need a fresh ignored copy.

e1 at 2026-01-15T10:00:00+10:00 becomes 00:00:00Z. Its clock is 30 seconds fast, so the corrected centre is 2026-01-14T23:59:30Z; +/-5 seconds gives 23:59:25Z through 23:59:35Z. e2 centres on 23:59:45Z with +/-3 seconds, giving 23:59:42Z through 23:59:48Z. The seven-second gap between e1 upper and e2 lower supports definitely-before for e1-vs-e2. e3 centres on 00:00:20Z with +/-2 seconds, so e2-vs-e3 is also definitely-before. None of these comparisons says the earlier event caused the later event.

The reference decision fields for guided are:

```json
{
  "e1-vs-e2": "definitely-before",
  "e2-vs-e3": "definitely-before"
}
```

Now open `python lab.py analyse --case run/worked-01/case.json` and compare observations, not only labels. Record one difference between your first prediction and the reference. If there was no difference, explain the rule that produced each decision rather than copying the label.

Handover prompt: explain what this evidence establishes, cite its exact IDs or lines, give a plausible alternative or remaining uncertainty, and name the smallest next check. Record any help used. A correct label by itself is not evidence of independent understanding.

# Drills: Timeline normalisation and uncertainty

## Guided practice

Prepare guided in a fresh run folder, read the README rules, and make a prediction for every decision ID. Use WORKED-EXAMPLE.md only after saving that prediction. Correct your explanation in a different paragraph so the original reasoning remains visible.

## Prompts only

Close the worked example and use these questions to reconsider the evidence:

- Why do you subtract a positive device-clock offset?
- Which two bounds must be compared to establish definitely-before?

## Independent attempt

Prepare challenge in a fresh folder. Keep reference outputs, worked example and analyse closed until you have saved answers.json plus a five-sentence handover in notes.txt. Include the observation, interpretation, an alternative, uncertainty and next check. Use the README for syntax if needed and record that help. Afterwards run analyse/check and have a person inspect the explanation. Independent completion is unassessed unless someone actually reviews it.

## Changed case

Prepare changed. Predict which conclusions change and which do not. Explain the changed evidence before checking:

- Why does a wider uncertainty interval remove the first ordering conclusion?
- Does exactly one common endpoint allow strict ordering?

Small edit drill: Change only e1 uncertainty_seconds from 5 to 12 in a copied guided case. Its upper bound becomes 23:59:42Z and touches e2; predict the comparison before analysis.

## Teach-back

Show the UTC conversion, offset correction and interval comparison separately, then state what remains unknown. Use exact record IDs from your attempt. A partner should ask one counterexample rather than accepting a memorised label. Record the question and your response.

## Later recall

After 24-72 hours, reopen only your own notes and a fresh case. Write the central rule from memory, identify one boundary or missing-evidence condition and predict the result. Then check the documentation and record what you forgot. Do not count rereading an answer as an independent attempt.

## Handover

Keep the copied case, completed worksheet and notes together. State the scope, conclusion, two evidence references, unresolved uncertainty and requested next check. Include help used and whether someone assessed the reasoning. Do not claim experience beyond this fictional exercise.