Unix Timestamp vs ISO 8601: Which Date Format Should You Use?

When building applications, choosing the right date-time format is a decision that affects everything from API design to database performance. The two dominant formats in modern software are Unix timestamps (also called epoch time) and ISO 8601 (the international standard for date-time strings). Each has distinct strengths and trade-offs that make it better suited for different use cases.

At a Glance

Detailed Comparison

Understanding Unix Timestamps

A Unix timestamp (or epoch time) represents a point in time as the number of seconds that have elapsed since January 1, 1970, 00:00:00 UTC, known as the Unix epoch. This format was introduced in early Unix operating systems and has become a de facto standard across all programming languages and platforms.

The key advantage of Unix timestamps is their simplicity. Because time is represented as a single number, calculating durations is trivial: subtract one timestamp from another to get the elapsed seconds. Comparing two timestamps is a simple numeric comparison, which is among the fastest operations a CPU can perform. This makes Unix timestamps ideal for high-performance applications, database indexing, and any scenario where you need to perform arithmetic on time values.

However, Unix timestamps are opaque to humans. The number 1705312200 conveys nothing without a converter. They also lack built-in timezone context -- though this is often considered a feature, since it eliminates timezone ambiguity.

Understanding ISO 8601

ISO 8601 is an international standard for representing dates and times as strings. The most common format is YYYY-MM-DDTHH:mm:ssZ, where the "T" separates date from time and "Z" indicates UTC. The standard also allows timezone offsets like +05:30 or -08:00.

The greatest strength of ISO 8601 is readability. A developer reading a log file or API response can immediately understand the date and time without any tools. It is also self-documenting: the format includes timezone information, making it clear whether a timestamp is in UTC or a local timezone. RFC 3339, which is a profile of ISO 8601, is the standard format for timestamps in JSON APIs, HTTP headers, and many web protocols.

An often-overlooked advantage of ISO 8601 is that its lexicographic sort order matches its chronological order (when using the same timezone offset). This means you can sort ISO 8601 strings as plain text and get correct time ordering, which is useful in log files and file naming conventions.

When to Use Each Format

Code Examples: Converting Between Formats

// Unix timestamp to ISO 8601
const timestamp = 1705312200;
const iso = new Date(timestamp * 1000).toISOString();
// "2024-01-15T10:30:00.000Z"

// ISO 8601 to Unix timestamp
const isoString = "2024-01-15T10:30:00Z";
const unix = Math.floor(new Date(isoString).getTime() / 1000);
// 1705312200

// Current time in both formats
console.log(Math.floor(Date.now() / 1000));  // Unix
console.log(new Date().toISOString());        // ISO 8601

from datetime import datetime, timezone

# Unix timestamp to ISO 8601
timestamp = 1705312200
iso = datetime.fromtimestamp(timestamp, tz=timezone.utc).isoformat()
# "2024-01-15T10:30:00+00:00"

# ISO 8601 to Unix timestamp
iso_string = "2024-01-15T10:30:00+00:00"
unix = int(datetime.fromisoformat(iso_string).timestamp())
# 1705312200

# Current time in both formats
import time
print(int(time.time()))                                      # Unix
print(datetime.now(timezone.utc).isoformat())                # ISO 8601

-- Unix timestamp to ISO 8601
SELECT to_char(
  to_timestamp(1705312200),
  'YYYY-MM-DD"T"HH24:MI:SS"Z"'
);
-- "2024-01-15T10:30:00Z"

-- ISO 8601 to Unix timestamp
SELECT EXTRACT(EPOCH FROM '2024-01-15T10:30:00Z'::timestamptz)::int;
-- 1705312200

Common Pitfalls to Avoid

Frequently Asked Questions