src/Providers/None.php
public function getTimestamp()
    {
        if (!$this->isGitRepo()) {
            return '';
        }

        $timestamp = exec('git log --pretty=format:"%ai" -1');
        $timestamp = preg_replace('/ /', 'T', $timestamp, 1);
        $timestamp = str_replace(' ', '', $timestamp);
        $timestamp = preg_replace('/([0-5]{2})([0-5]{2})$/', '$1:$2', $timestamp);

        return $timestamp;
    }
public function getTimestamp()
    {
        if (!$this->isGitRepo()) {
            return '';
        }

        $timestamp = exec('git log --pretty=format:"%ai" -1');
        $timestamp = preg_replace('/ /', 'T', $timestamp, 1);
        $timestamp = str_replace(' ', '', $timestamp);
        $timestamp = preg_replace('/([0-5]{2})([0-5]{2})$/', '$1:$2', $timestamp);

        return $timestamp;
    }
Git has an exact ISO 8601 format (only since August 2014): %aI.
However, since it's relatively young, it isn't yet available
everywhere. There's another very similar format, though: %ai.

This is an example of what %ai gives, and what it should look like:
Current: 2016-02-18T16:02:46+01:00
Correct: 2016-02-18 16:02:46 +0100
Close enough. Let's just transform it to how we want it to look...

@see https://github.com/git/git/commit/466fb6742d7fb7d3e6994b2d0d8db83a8786ebcf

{@inheritdoc}