Unix Timestamp

What is a Unix Timestamp?

A Unix timestamp (also called Epoch time or POSIX time) is simply the number of seconds since January 1, 1970, 00:00:00 UTC. That date is called the "Unix Epoch."

Right now: 1706745600
Meaning:   Fri Jan 31 2024 00:00:00 UTC

No timezones. No formatting. Just a number that every system understands.

Why 1970?

Unix was being developed in the late 1960s. The engineers needed a starting point for time and picked a round number close to when they were working. January 1, 1970 was convenient—no one was going to need dates from the 1960s in their software (they thought).

Format Variations

Where You'll See This

• APIs - Created/updated timestamps in responses
• Databases - Storing dates as integers (fast, compact)
• JWTs - iat (issued at), exp (expires) claims
• Logs - Timestamped events
• Caching - Cache expiration times
• Git - Commit timestamps

Advantages

The Year 2038 Problem

Unix timestamps are traditionally stored as a signed 32-bit integer. The maximum value is 2,147,483,647, which corresponds to:

January 19, 2038, 03:14:07 UTC

After that, 32-bit systems will overflow and wrap to negative numbers (interpreted as 1901). This is the "Y2K38" problem.

Converting

// Current timestamp
const now = Math.floor(Date.now() / 1000);

// Timestamp to Date
const date = new Date(1706745600 * 1000);

// Date to timestamp
const timestamp = Math.floor(new Date('2024-01-31').getTime() / 1000);

// Readable format
new Date(1706745600 * 1000).toISOString();
// "2024-01-31T16:00:00.000Z"

Common Operations

// Add 1 hour
const future = timestamp + 3600;

// Subtract 1 day
const yesterday = timestamp - 86400;

// Check if expired
const isExpired = Date.now() / 1000 > expirationTimestamp;

// Time until expiration
const secondsLeft = expirationTimestamp - Date.now() / 1000;

Common Gotchas

• Seconds vs milliseconds - JavaScript uses ms, Unix uses seconds. Don't forget to convert.
• Timezone confusion - Timestamps are always UTC. Convert to local time for display only.
• Leap seconds - Unix time ignores them. 86,400 seconds is always one day.
• Negative timestamps - Dates before 1970 are negative numbers.

Unix Time vs ISO 8601

Try It

"Time is an illusion. Unix time doubly so." — with apologies to Douglas Adams