Varying date values in Frontmatter between local PHP and GitHub Action

[robindotis]

There seems to be a difference in how frontmatter dates are represented between my local PHP install and when running on GitHub actions.

My frontmatter looks something this

---
title: My first post
date: 2019-12-30 00:00:00 Z
tags:
- work
- web
---

I am reading the frontmatter using this command, which inside a loop going through all my posts in markdown format:

use League\CommonMark\Extension\FrontMatter\Output\RenderedContentWithFrontMatter;
.
.
.
            $frontMatter = [];
            if ($result instanceof RenderedContentWithFrontMatter) {
                $frontMatter = $result->getFrontMatter();
            }
            echo "\n";
            echo $frontMatter['date'];

The echo statement gives different output when run on my local machine compared to GitHub Actions.

On my local machine I get timestamps:

946598400
965952000
966038400
966124800
965606400

But on GitHub I get the actual string from the frontmatter YAML:

2019-12-30 00:00:00 Z
2019-03-09 00:00:00 Z
2019-12-02 00:00:00 Z
2019-06-26 00:00:00 Z
2019-01-24 00:00:00 Z

I don't understand why there is this difference. I would expect the output to be as with the GitHub Action.

Could this be a config setting on my local machine?

Any ideas would be welcome. Thanks.