DJ-73: migrate backend off joda-time to java.time and show relative timestamps in the frontend

[devin-ai-integration]

Summary

Removes the deprecated joda-time dependency from the backend (migrating to java.time.Instant) and replaces the frontend's static toDateString() timestamps with relative times plus a full-date tooltip. DJ-73.

The constraint that shaped the backend work: the API date contract had to stay byte-identical. Joda's ISODateTimeFormat.dateTime().withZoneUTC() always emits 3-digit millis (2026-07-29T20:57:31.000Z), while Instant.toString() drops trailing-zero millis (...31Z), so every serialization path goes through one explicit formatter instead:

// io.spring.application.DateTimeFormats
public static final DateTimeFormatter DATE_TIME_FORMAT =
    DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'").withZone(ZoneOffset.UTC);

used by JacksonCustomizations.DateTimeSerializer (now StdSerializer<Instant>) and by both GraphQL datafetchers, replacing the per-call ISODateTimeFormat...print(...). Everything else is a type swap: Article/Comment/ArticleData/CommentData fields, CursorPageParameter<DateTime> → <Instant>, and DateTimeHandler becomes TypeHandler<Instant> (@MappedTypes(Instant.class)) keeping the identical UTC-Calendar/Timestamp round-trip. DateTimeCursor keeps emitting raw epoch millis (getMillis() → toEpochMilli(), new DateTime().withMillis(x) → Instant.ofEpochMilli(x)).

Frontend adds frontend/lib/utils/relativeTime.ts — no new runtime deps, just Intl.RelativeTimeFormat — exporting getRelativeTime(iso, now?) (largest sensible unit, "just now" under 10s, "" for invalid/empty input) and getFullDate(iso) for the tooltip:

- <span className="date">{new Date(preview.createdAt).toDateString()}</span>
+ <span className="date" title={getFullDate(preview.createdAt)}>
+   {getRelativeTime(preview.createdAt)}
+ </span>

ArticleType.createdAt/updatedAt were typed number while the API has always returned ISO strings; corrected to string.

TypeScript moves ^3.9.7 → 4.9.5 because Intl.RelativeTimeFormat typings don't exist before 4.1 — on 3.9.10 the build fails with error TS2339: Property 'RelativeTimeFormat' does not exist on type 'typeof Intl'. jest/ts-jest/@types/jest are added as dev deps for the new unit tests.

Tests

New unit tests: DateTimeFormatsTest (asserts whole-second instants serialize as ...:31.000Z, not ...:31Z, plus a .123Z case, over both the Jackson and datafetcher paths), DateTimeCursorTest (cursor string == epoch millis, encode→decode round-trip), DateTimeHandlerTest (symmetric UTC Timestamp conversion), and frontend/lib/utils/relativeTime.test.ts (17 cases: every unit boundary, the exact .000Z API format, future dates, invalid/empty input, tooltip helper).

Verification: ./gradlew clean test spotlessCheck passes (72 tests, 0 failures; jacocoTestCoverageVerification fails at 0.33 vs 0.80 both here and on unmodified main, so it is pre-existing and untouched). Frontend: npm test 17/17, npx tsc --noEmit clean, next build compiles. grep -rn joda src/ build.gradle is empty.

Live check against the running app — curl http://localhost:8080/articles still returns "createdAt":"2026-07-29T21:26:19.000Z", matching ^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$ for every article, same field shape as the pre-migration capture.

Home feed: before / after

Before (static dates)	After (relative times)

Full date on hover:

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/6c3903acf9244ec4b20cd69300aede02
Requested by: @HJscarr

[devin-ai-integration]

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