JS Foundations #13 — Date & Time

JavaScript's Date object handles timestamps, formatting, and time arithmetic. It's used in interviews for timing (throttle, debounce), timestamp generation, and date manipulation. This covers the practical methods you'll reach for.

1. Creating Dates

Month is 0-based — 0 = January, 11 = December. This is the most common Date pitfall.

2. Getting Timestamps

Interview use: Date.now() in throttle (#9) to check if enough time has elapsed since the last call.

3. Getting Date Components

Always prefer UTC methods to avoid timezone surprises. Store and transmit dates in ISO 8601 / UTC.

4. Setting Date Components

Setters mutate the Date object. They auto-correct overflow: setDate(0) goes to the last day of the previous month.

5. Date Arithmetic

Subtract dates to get milliseconds; divide to get other units:

6. Formatting Dates

ISO 8601 (Preferred for Storage/Transmission)

Locale-Aware Formatting

Human-Readable

7. performance.now() — High-Resolution Timing

For measuring code execution time, use performance.now() instead of Date.now():

performance.now() returns milliseconds with microsecond precision (fractional) and is monotonic — it never goes backwards, unlike system clock changes.

Interview use: Measuring algorithm runtime in complexity analysis.

8. Parsing Dates Reliably

9. Checking Validity

Quick Reference

Interview Tips

• Use Date.now() for timing — no need to create a Date object for a timestamp.
• Use performance.now() for benchmarks — higher precision and monotonic.
• Use UTC methods for consistency — getUTCFullYear(), not getFullYear().
• Remember month is 0-based — new Date(2024, 0) is January, not December.

Related Articles

• #8 — Implement debounce()
• #9 — Implement throttle()
• #28 — Deep Clone with Circular References