DJ-73: Migrate backend off Joda-Time to java.time + relative frontend timestamps

[devin-ai-integration]

Summary

Pays down the tech debt in DJ-73 in two workstreams merged into one branch:

Backend — joda-time removed entirely. joda-time:2.10.13 dropped from build.gradle; all org.joda.time.DateTime usages replaced with java.time.Instant across the domain (Article, Comment), DTOs (ArticleData, CommentData), query services, GraphQL datafetchers, the MyBatis DateTimeHandler, DateTimeCursor, and tests. The API date contract is preserved byte-for-byte: JacksonCustomizations now serializes Instant with

DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'").withZone(ZoneOffset.UTC)

which emits exactly the same ISO-8601 output as joda's ISODateTimeFormat.dateTime().withZoneUTC() (millisecond precision + literal Z, e.g. 2026-07-29T20:29:12.000Z) — verified by diffing curl http://localhost:8080/articles against a pre-migration baseline (identical after normalizing reseeded timestamp values). DateTimeCursor still round-trips epoch millis and DateTimeHandler keeps the same UTC-calendar Timestamp storage, so pagination and existing SQLite data behave unchanged.

Frontend — relative timestamps. New frontend/lib/utils/relativeTime.ts using the built-in Intl.RelativeTimeFormat (no new dependencies; "just now" under 45s, otherwise the largest sensible unit). ArticlePreview, ArticleMeta, and Comment now render relative times (e.g. "2 days ago") with the absolute date in a title tooltip.

Before / After (home feed)

Before (static dates)	After (relative timestamps)

Verification

• ./gradlew clean test spotlessCheck: 68 tests pass, spotless clean. jacocoTestCoverageVerification fails at 0.33 vs the 0.80 gate — preexisting on main (0.3315 baseline vs 0.3322 on this branch, i.e. slightly improved).
• Frontend npm run build passes.
• grep -ri joda build.gradle src returns nothing.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/b9fa4a7174334d65b2e60d863b4e5843
Requested by: @HJscarr

[devin-ai-integration]

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