Seconds vs Milliseconds Timestamps

One of the most common sources of timestamp bugs is confusing seconds-based and milliseconds-based timestamps. A seconds timestamp like 1700000000 represents November 14, 2023, while the same digits with three trailing zeros (1700000000000) as a millisecond timestamp represents the same moment, just with finer precision.

How to tell them apart:

The simplest heuristic is digit count. As of the 2020s, a seconds-based Unix timestamp has 10 digits, while a milliseconds timestamp has 13. This rule holds reliably from the year 2001 through 2286.

10 digits → seconds      (e.g., 1700000000)
13 digits → milliseconds (e.g., 1700000000000)
16 digits → microseconds (e.g., 1700000000000000)
19 digits → nanoseconds  (e.g., 1700000000000000000)

Which languages use which:

Unit	Languages/Systems
Seconds	Python, PHP, Ruby, Bash, Go, C, MySQL
Milliseconds	JavaScript, Java, Dart, C# (DateTimeOffset)

Conversion between them:

// Milliseconds to seconds
const seconds = Math.floor(milliseconds / 1000);

// Seconds to milliseconds
const ms = seconds * 1000;

Common bug scenario: You store a JavaScript Date.now() value (milliseconds) in a database and then read it in a Python script that expects seconds. The Python code interprets it as a date tens of thousands of years in the future. The fix is to always document your timestamp unit at the API boundary and convert consistently.

API design tip: When designing APIs, explicitly state the unit in your field name (e.g., created_at_ms or expires_at_unix_seconds). This prevents ambiguity and saves hours of debugging for consumers of your API. Many modern APIs, including Stripe and Twilio, use seconds-based timestamps and document this clearly.