feat(template): Add addDuration template helper for time offsets

[holger-waschke]

Summary
Adds an addDuration helper function to Alertmanager templates. This allows templates to shift timestamps using Go duration strings (e.g., "-30m", "1h") and returns the resulting value as Unix milliseconds for direct use in links.

Why?
Before this change, achieving relative time windows required raw nanosecond arithmetic, which is hard to read and error-prone:

{{- $start := (.StartsAt.Add -1800000000000).UnixMilli -}}
{{- $end   := (.StartsAt.Add 3600000000000).UnixMilli -}}

With addDuration, the same logic becomes much more expressive and maintainable:

{{- $start := (addDuration .StartsAt "-30m") -}}
{{- $end   := (addDuration .StartsAt "60m") -}}
{{- $url   := printf "https://app.checklyhq.com/checks/%s?startTime=%d&endTime=%d" .Labels.check_id $start $end -}}
[Checkly Playwright Report|{{ $url }}]

Changes

• Registered addDuration in the default template function map.
• Added unit tests covering:
• Valid duration offset parsing ("-30m", "1h").
• Error handling for invalid duration strings.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: ec8aad9c-a29f-492c-8cd4-268c47c3236b

📥 Commits

Reviewing files that changed from the base of the PR and between 027f11a and c1fbbad.

📒 Files selected for processing (2)

• template/template.go
• template/template_test.go

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

The template package adds an addDuration helper. It parses duration strings, adds valid durations to time.Time values, and returns Unix timestamps in milliseconds. Tests cover valid and invalid inputs.

Changes

Template duration support

Layer / File(s)	Summary
Add and validate duration calculation template/template.go, template/template_test.go	The addDuration helper parses durations, returns errors for invalid input, and returns adjusted timestamps in Unix milliseconds. Tests cover valid output and the exact invalid-input error.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: 🟡 Moderate · up to c1fbb

Templates using standard pipeline syntax may fail to render when calling the new timestamp-shifting helper. Correct the argument handling and add a pipeline regression test before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description explains the purpose, motivation, implementation changes, usage example, and tests. However, it omits the required Pull Request Checklist, issue information, documentation and sign-off…	Add the required Pull Request Checklist with all applicable boxes marked, list related issues or state that none apply, provide the required documentation and sign-off confirmations, and complete the release-notes block with NONE or an appr…

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the template area and the new addDuration helper. It follows the repository's requested area-and-description format.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description explains the purpose, motivation, implementation changes, usage example, and tests. However, it omits the required Pull Request Checklist, issue information, documentation and sign-off confirmations, best-practices confirmation, and release-notes section.

Resolution

Add the required Pull Request Checklist with all applicable boxes marked, list related issues or state that none apply, provide the required documentation and sign-off confirmations, and complete the release-notes block with NONE or an appropriate user-facing entry.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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