It might seem obvious but I found out that a lot of people don’t know you can sync your Apple iPhone with your Google calendar.
So here is a very short how-to.

In order to sync your iPhone’s calendar with your google calendar you must create a new Microsoft Exchange account under Preferences:

1. Open the Settings application on your iPhone.
2. Open Mail, Contacts, Calendars.
3. Tap Add Account.

Now add your account settings:

1. In the Email field, enter your full Google Account email address.
2. Leave the Domain field blank.
3. Enter your full Google Account email address as the Username.
4. Enter your Google Account password as the Password.
5. Tap Next at the top of your screen.
6. When the new Server field appears, enter m.google.com.
7. Press Next at the top of your screen again.

If you have multiple calendars you have to visit m.google.com/sync with your iPhone’s browser and select which calendars you want see in the calendar application.
Be careful in giving appointments quickly, since it takes a few seconds to update your phone’s calendar, unless you have Push enabled.