Day 30 of 100: Dates and Times — Timestamps, Durations, Comparisons

Day 30 of 100 — 100 Days of Playwright with TypeScript Challenge. Phase: JavaScript Fundamentals. JavaScript (Days 1-33) → TypeScript (Days 34-63) → Playwright (Days 64-96) → AI Framework Capstone (Days 97-100).

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Test data needs timestamps. Reports need durations. Assertions compare dates. The Date API handles all three.

Creating and Reading

const now = new Date();
Date.now();                       // epoch ms — great for unique IDs
now.toISOString();                // '2026-09-30T04:30:00.000Z' — API standard

const due = new Date('2026-12-31');
due.getFullYear();                // 2026
due.getMonth();                   // 11 — ZERO-INDEXED! Dec = 11

QA Patterns

// Unique test data names
const runId = `run-${Date.now()}`;

// Duration measurement
const t0 = Date.now();
// ... await actions ...
const elapsed = Date.now() - t0;
console.log(`took ${elapsed}ms`);

// Relative dates for test data
const tomorrow = new Date(Date.now() + 24 * 60 * 60 * 1000);
const lastWeek = new Date(Date.now() - 7 * 86400000);

// Comparing API dates
const created = new Date('2026-09-30T10:00:00Z');
const updated = new Date('2026-09-30T11:00:00Z');
updated > created;               // true — direct comparison works

Gotchas

• getMonth() is 0-indexed — December is 11
• Date parsing without timezone assumes local — use ISO strings with Z
• Never compare formatted strings — compare Date objects or epoch numbers

Tomorrow (Day 31): Sets and Maps — deduplication and lookups.