Unix Timestamp Converter

What Is a Unix Timestamp?

A Unix timestamp (also known as Unix time, POSIX time, or epoch time) is the number of seconds that have elapsed since January 1, 1970, at 00:00:00 Coordinated Universal Time (UTC). This moment is known as the Unix epoch. Unix timestamps provide a simple, unambiguous way to represent a specific moment in time as a single integer, making them the universal standard for recording time in computer systems, databases, APIs, and log files.

The Unix Epoch: Why January 1, 1970?

The choice of January 1, 1970, as the epoch was an early design decision by the creators of Unix at Bell Labs. The original Unix system stored time as a 32-bit signed integer counting seconds from the epoch. This was a practical choice that provided a simple, monotonically increasing counter that would be valid for decades. At the time of its creation in the early 1970s, the epoch was set to a recent, round date. The decision proved remarkably durable, and the Unix epoch is now used far beyond Unix systems, including Windows, web browsers, mobile operating systems, databases, and virtually every programming language.

The Year 2038 Problem (Y2K38)

The original 32-bit signed integer representation of Unix time can store values up to 2,147,483,647, which corresponds to January 19, 2038, at 03:14:07 UTC. After this moment, a 32-bit counter overflows and wraps to a negative number, representing a date in December 1901. This is known as the Y2K38 problem or the Unix Millennium Bug. Modern systems have largely mitigated this by using 64-bit integers for timestamps, which extends the range to approximately 292 billion years from the epoch. However, legacy embedded systems, older databases, and some file formats that still use 32-bit timestamps remain vulnerable.

Seconds vs. Milliseconds

The traditional Unix timestamp is measured in seconds. However, many modern systems, particularly JavaScript and Java, use millisecond-precision timestamps (the number of milliseconds since the epoch). A millisecond timestamp is simply the second-based timestamp multiplied by 1000. You can easily distinguish the two by length: a seconds timestamp is currently 10 digits (for example, 1700000000), while a milliseconds timestamp is 13 digits (for example, 1700000000000). This converter automatically detects which format you have entered and handles the conversion accordingly.

Unix Timestamps in Programming Languages

Every major programming language has built-in support for Unix timestamps. In JavaScript, Date.now() returns milliseconds and Math.floor(Date.now() / 1000) returns seconds. In Python, import time; time.time() returns seconds as a float. In PHP, time() returns seconds. In Java, System.currentTimeMillis() returns milliseconds. In C / C++, time(NULL) returns seconds. In Go, time.Now().Unix() returns seconds. In Ruby, Time.now.to_i returns seconds. In Rust, SystemTime::now().duration_since(UNIX_EPOCH) gives you the duration in seconds and nanoseconds. Understanding these differences is essential when working with APIs that may return timestamps in different precisions.

Common Date Formats Explained

ISO 8601 is the international standard for date and time representation (for example, 2024-01-15T10:30:00.000Z). The trailing Z indicates UTC. This format is unambiguous, sortable as a string, and the recommended format for APIs and data exchange. RFC 2822 is used in email headers and HTTP (for example, Mon, 15 Jan 2024 10:30:00 +0000). The US format uses month-first ordering (January 15, 2024), while the EU format uses day-first ordering (15/01/2024). When building software, always store and transmit dates as Unix timestamps or ISO 8601 strings, and convert to localized display formats only at the presentation layer.

Negative Timestamps

Unix timestamps can be negative, representing dates before the epoch. For example, a timestamp of -86400 represents December 31, 1969 (one day before the epoch). This is useful for historical data. However, not all systems handle negative timestamps correctly, so test thoroughly if your application needs to work with dates before 1970.