The PHP Intl class is really very handy for an i18n site, allowing you to easily output the correct date formats depending on the end users locale.
If you start using it but get an error like this:
Fatal error: Class 'IntlDateFormatter' not found
Then you just need to install the Intl extension.
Fortunately this is dead easy. Just run the following commands
sudo apt-get install php5-intl
You then need to restart Apache
sudo service apache2 restart
You can also check what extensions are compiled in by running