Symfony\Component\Yaml\Inline::evaluateScalar() uses Symfony\Component\Yaml\Inline::getTimestampRegex() to determine whether a scalar value contains a date in what seems to be the ISO 8601 format (getTimestampRegex() is badly documented) and if it does, it converts it to a Unix timestamp.
The problem is that this conversion is conceptually impossible for a number of reasons:

• ISO 8601 dates and Unix timestamps use different formats. Converting one to the other may break systems that expect a particular format.
• Unix timestamps do not support dates before 1970, while ISO 8601 does. This means data loss.

Suggested solution: remove the feature entirely.