JavaScript Date & Epoch: Complete Guide

Dates and times are tricky in frontend and backend apps. Understanding epoch, Date objects, instance methods, and static methods is essential for correct time computation, formatting, and transport.

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1️⃣ Epoch Time

Epoch time is the number of seconds or milliseconds since the Unix Epoch:

1970-01-01T00:00:00 UTC

• Milliseconds → JavaScript Date uses this internally

• Seconds → Common in Unix, logs, APIs

Example:

const now = new Date();
console.log(now.getTime());        // 1766750599479 (milliseconds)
console.log(Math.floor(now.getTime() / 1000)); // 1766750599 (seconds)

Rule of thumb:

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2️⃣ Date.now()

Static method that gives current timestamp in milliseconds.

console.log(Date.now()); // 1766750599479

• Does not require creating a Date object

• Useful for benchmarking, timers, logging

• Equivalent to (new Date()).getTime(), but faster

Example: Measuring execution time

const start = Date.now();
// some work
const end = Date.now();
console.log("Duration:", end - start, "ms");

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3️⃣ date.getTime()

Instance method for any Date object. Returns milliseconds since epoch.

const date = new Date("2025-12-26T12:03:19Z");
console.log(date.getTime()); // 1766750599479

• Use this when you have a specific date to compute differences, sort events, or store numeric timestamp.

• Works well for arithmetic between dates:

const start = new Date("2025-12-26T10:00:00Z");
const end = new Date("2025-12-26T12:00:00Z");

const diffMs = end.getTime() - start.getTime(); // 7200000 ms
const diffSeconds = diffMs / 1000;             // 7200 seconds

Key difference from Date.now():

• getTime() → timestamp of that specific Date object

• Date.now() → timestamp of the current instant

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4️⃣ Other Important Date Instance Methods

Tip: getUTC*() versions exist (e.g., getUTCHours()) to get UTC values instead of local.

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5️⃣ Other Important Date Static Methods

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6️⃣ When to use which

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7️⃣ Example: Full Flow

// Backend sends ISO string
const isoString = "2025-12-26T12:03:19.479Z";

// Frontend parses ISO string → Date object
const date = new Date(isoString);

// Current timestamp
const nowMs = Date.now();
console.log(nowMs);

// Timestamp of specific date
console.log(date.getTime());

// Compute difference in seconds
const diffSeconds = (nowMs - date.getTime()) / 1000;
console.log(diffSeconds);

// Format for user
console.log(new Intl.DateTimeFormat("en-GB", {
  dateStyle: "medium",
  timeStyle: "short",
  timeZone: "Europe/London"
}).format(date));

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8️⃣ Key Takeaways

1. Epoch milliseconds → JavaScript standard; Date stores this internally.

2. Epoch seconds → common in Unix systems or APIs.

3. Date.now() → current timestamp in milliseconds (static).

4. date.getTime() → timestamp for that specific Date object (instance).

5. Other instance methods → extract year, month, day, hour, minute, etc.

6. Other static methods → Date.UTC(), Date.parse().

7. Use ISO strings for transport, epoch for computation, Intl for display.