Gotcha: Jekyll post generated with wrong date

It is said that most of the hard problems of computer science have been solved, but two remain: cache invalidation, naming, and off-by-one errors.

Timezones may in theory be a solved problem (“‘just’ always work in UTC!”), but it’s one which still comes up annoyingly often.

Building locally

My previous deployment flow for this blog you’re reading right now was to build it locally with jekyll build, and use Octopress deploy to deploy it to a separate repo.

While this was in place, the previous blog post was correctly generated with a date of the 18th of February:

Building on CI

When I switched to using a Github action to deploy automatically on push, the post started getting generated with a date of the 17th:

This was pretty frustrating: the filename said the 18th:


…and the date in the Jekyll front-matter, (which I had set manually to be at midnight) was the 18th:

layout: post
title: "\"Somebody is upside down!\": building a bingo app in React"
date: 2022-02-18 00:00:00 +0100
comments: true
categories: [reactjs]
tags: [reactjs, react]

But observant readers will have spotted the problem:

Previously I was building the site on my local machine, which is in Sweden, where 2022-02-18 00:00:00 +0100 is midnight on the 18th.

But the Github action is running in UTC (as all good servers do), and in UTC 2022-02-18 00:00:00 +0100 is 23:00 on the 17th.

Changing the timestamp to midday fixes the issue: 2022-02-18 00:12:00 +0100.


I should know better: when there’s a problem involving differences between CI and local, particularly when the differences are to do with times, timezones are a very likely culprit.

I guess there are two morals to this story:

  1. don’t write blog posts close to midnight
  2. don’t arbitrarily set timestamps close to midnight

This sort of problem was less common when I lived in the UK (except in summer).