What is a Unix Timestamp?
A Unix timestamp (also called epoch time, POSIX time, or Unix time) is a system for tracking time as a running total of seconds. It counts the number of seconds that have elapsed since the Unix Epoch: January 1, 1970 at 00:00:00 UTC.
Unix timestamps are timezone-independent—they represent a single moment in time globally. This makes them ideal for storing, comparing, and computing with dates in software systems.
Understanding the Unix Epoch
January 1, 1970 00:00:00 UTC
Examples:
Timestamp 0 = Jan 1, 1970 00:00:00 UTC
Timestamp 86400 = Jan 2, 1970 00:00:00 UTC (+ 1 day)
Timestamp 1000000000 = Sep 9, 2001 01:46:40 UTC
Timestamp 2000000000 = May 18, 2033 03:33:20 UTC
Conversion Formulas
timestamp = Math.floor(Date.getTime() / 1000)
Unix Timestamp to Date:
date = new Date(timestamp * 1000)
Seconds vs Milliseconds:
10 digits = seconds (1704067200)
13 digits = milliseconds (1704067200000)
Seconds vs Milliseconds
Traditional Unix timestamps are in seconds. However, many modern systems (JavaScript, Java) use milliseconds for greater precision:
- Seconds (10 digits): Unix/Linux, PHP, Python's time.time(), MySQL UNIX_TIMESTAMP()
- Milliseconds (13 digits): JavaScript Date.now(), Java System.currentTimeMillis()
- Microseconds (16 digits): Some high-precision systems
- Nanoseconds (19 digits): Go's time.Now().UnixNano()
The Year 2038 Problem
On January 19, 2038 at 03:14:07 UTC, 32-bit signed integer timestamps will overflow, wrapping to negative numbers (interpreted as December 1901). This affects:
- Older 32-bit systems and embedded devices
- Legacy software using 32-bit integers for time
- Some databases with limited timestamp fields
64-bit systems avoid this problem and can represent dates billions of years in the future.
Benefits of Unix Timestamps
- Timezone independent: Always UTC, no timezone confusion or DST issues.
- Easy arithmetic: Add/subtract seconds directly. Duration = end - start.
- Compact storage: Single integer vs. multiple date/time fields.
- Universal support: Every programming language and database supports them.
- Sortable: Chronological order = numerical order.
Frequently Asked Questions
Why does Unix time start from 1970?
The Unix operating system was developed at Bell Labs in the late 1960s. When defining the time system, they needed a fixed reference point. 1970 was recent enough to avoid large numbers for contemporary dates and early enough to predate the system itself.
Do Unix timestamps include leap seconds?
No, Unix timestamps ignore leap seconds. Days are always exactly 86,400 seconds. This means Unix time can differ from actual UTC by up to ~27 seconds (as of 2024). For most applications, this doesn't matter.
How do I get the current timestamp in different languages?
JavaScript: Math.floor(Date.now()/1000) | Python: import time; time.time() | PHP: time() | Java: System.currentTimeMillis()/1000 | SQL: UNIX_TIMESTAMP()
Can Unix timestamps be negative?
Yes, negative timestamps represent dates before January 1, 1970. For example, -86400 is December 31, 1969. However, some systems don't support negative timestamps.
How precise are Unix timestamps?
Standard Unix timestamps (seconds) have 1-second precision. Millisecond timestamps allow precision to 0.001 seconds. For sub-millisecond precision, use microsecond or nanosecond timestamps.
What's the maximum Unix timestamp?
32-bit signed: 2,147,483,647 (Jan 19, 2038). 64-bit signed: ~292 billion years in the future—effectively unlimited for any practical purpose.