Steps to reproduce:

1. Create a content type with a Datetime field definition, on platform ui admin interface;

2. Create a content of this content type, giving it a time value, and publish it;

3. Open your php.ini, and edit the entry, for instance:
date.timezone = "America/Puerto_Rico"
instead of whichever entry value you have there already;

4. Display this content on frontend.
The time is displayed in the timezone you set on php.ini.