Epoch / Unix Timestamp Format Guide

Unix Epoch Timestamps

A Unix timestamp (also called epoch time or POSIX 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.

Seconds vs Milliseconds

Two common variants exist:

1772236800        # Seconds (10 digits) — traditional Unix
1772236800000     # Milliseconds (13 digits) — JavaScript, Java

When you encounter a large integer representing time, the digit count tells you the precision:

• 10 digits — seconds (standard Unix)
• 13 digits — milliseconds (JavaScript Date.now(), Java System.currentTimeMillis())
• 16 digits — microseconds (Python time.time_ns() // 1000)
• 19 digits — nanoseconds (Go time.Now().UnixNano())

Advantages

1. Timezone-neutral — A timestamp represents an absolute moment regardless of timezone
2. Compact — A single integer is smaller than any formatted string
3. Sortable and comparable — Simple numeric comparison
4. Language-agnostic — Every language can work with integers

The Y2K38 Problem

Timestamps stored as 32-bit signed integers overflow on January 19, 2038 at 03:14:07 UTC (2,147,483,647 seconds after epoch). This is analogous to Y2K but for Unix systems. Modern systems use 64-bit integers, which won't overflow for approximately 292 billion years.

Conversion Across Languages

Language	To timestamp	From timestamp
JavaScript	Math.floor(Date.now() / 1000)	new Date(ts * 1000)
Python	int(time.time())	datetime.fromtimestamp(ts)
Java	Instant.now().getEpochSecond()	Instant.ofEpochSecond(ts)
PHP	time()	date('Y-m-d', $ts)
Go	time.Now().Unix()	time.Unix(ts, 0)
Ruby	Time.now.to_i	Time.at(ts)
C#	DateTimeOffset.UtcNow.ToUnixTimeSeconds()	DateTimeOffset.FromUnixTimeSeconds(ts)

Negative Timestamps

Dates before the epoch (pre-1970) are represented as negative numbers. For example, January 1, 1969 00:00:00 UTC is -31536000.