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.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository: prometheus/alertmanager/.coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: 29b8ff1c-fcfe-4620-a720-22b6a408dba3

📥 Commits

Reviewing files that changed from the base of the PR and between b28b549 and 86aaf2f.

📒 Files selected for processing (2)

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

🚧 Files skipped from review as they are similar to previous changes (1)

• template/template.go

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📝 Walkthrough

Walkthrough

The template package adds an addDuration helper and changes YAML-rendered string handling. The helper returns Unix milliseconds or a parse error. DeepCopyWithTemplate conditionally decodes valid JSON strings.

Changes

Template behavior updates

Layer / File(s)	Summary
Add and validate duration calculation template/template.go, template/template_test.go	The addDuration helper parses durations, adds valid durations to time.Time values, and returns Unix timestamps in milliseconds. Tests cover valid output and the exact invalid-input error.
Preserve rendered string values template/template.go	DeepCopyWithTemplate returns a decoded string scalar only when the rendered value is valid JSON. Other rendered strings remain unchanged, while structured YAML values continue through normalization.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 86aaf

The helper adds duration offsets for template-generated links and reports invalid durations safely. Current evidence supports merging.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding the addDuration template helper.
Description check	✅ Passed	The description explains the feature, motivation, usage, implementation changes, and test coverage. It does not include the repository checklist or release-notes entry, but the core required change in…
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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[TheMeier]

So if you flip the args addDuration(ds string, t time.Time) then both template forms would work

{{ addDuration "-30m" .StartsAt }}
{{ .StartsAt | addDuration "-30m" }}

If I read the coderabbit comment correctly

[fluktuid]

@TheMeier If I understand your comment correctly, this is how it should look for best compatibility.

@holger-waschke hope this helps