Why Did My JavaScript Timestamp Show the Wrong Date? A Dayjs Debugging Guide

Introduction

A few days ago a tester reported a bug: the time component displayed an abnormal date. The data was initialized from the backend, so I asked the backend whether the timestamp was wrong. They said the format had always been a numeric timestamp and never caused issues.

Comparing production data revealed the root cause: the backend changed the timestamp type from Number to String .

Bug Cause

The issue lies in how the timestamp is parsed. In the code I consistently use dayjs for date parsing.

Typical parsing code:

<code>const time = dayjs(res.endTime).format('YYYY-MM-DD HH:mm:ss')</code>

I tried parsing both string and numeric forms:

String: dayjs('175008959900').format('YYYY-MM-DD hh:mm:ss') // 1975-07-19 01:35:59

Number: dayjs(Number('175008959900')).format('YYYY-MM-DD HH:mm:ss') // 2025-07-17 06:59:59

Because the backend now returns a string like '175008959900' , dayjs() treats it as a generic date string and does not automatically recognize it as a millisecond timestamp, leading to invalid parsing or fallback to the Unix epoch.

How to Avoid This Issue

Just like new Date() , both Date and dayjs should always receive a numeric timestamp. Convert the input defensively:

<code>const ts = Number(res.endTime);
const date = new Date(ts);
</code>

Reflection

A timestamp is a numeric representation of time, usually counting the elapsed milliseconds (or seconds) since the Unix epoch (1970‑01‑01 00:00:00 UTC).

Common timestamp types:

Unix timestamp (seconds) – e.g., 1750089599 – typical in backend APIs and databases.

Millisecond timestamp – e.g., 1750089599000 – common in JavaScript (e.g., Date.now() ).

Significance of timestamps:

Provides an absolute numeric representation of time that is language‑ and platform‑agnostic.

Enables easy arithmetic: subtracting two timestamps yields the interval in milliseconds.

More compact and performant than formatted date strings.

Using timestamps in JavaScript:

<code>console.log(Date.now()); // e.g., 1714729530000
console.log(new Date(1750089599000)); // Thu Jul 17 2025 06:59:59 GMT+0800
</code>