How do I add contacts' Birthdays to Google Calendar?

You can add your contact's birthdays as an additional calendar in your Google calendar via their web interface. DigiCal will then display all your contact's birthdays accordingly.

Adding contacts' Birthdays to your Google Calendar

Google Calendar lets you add yearly recurring events; handy to see friends' birthdays in advance, for example. Additionally, Gmail lets you add dates to your contacts; handy to note friends' birthdays. 

Conveniently, Google Calendar can automatically pick up birthdays you've added to your Gmail contacts and show them as a special schedule of all-day events — even including birthday cakes icons!

Add Your Gmail Contacts' Birthdays to Google Calendar

• In the top right, click Settings.
• Under "settings for my calendars" on the left, make sure Contacts/Birthdays is not already listed as an option. If it is then check the "eye" icon to the right of the calendar name and make sure it's set to "show in calendar list".
• If it's not listed then you need to add it first. Click Add Calendar > Browse calendars of interest, and check the box called Your Contacts. The new calendar named Contacts/Birthdays will then be shown under "settings for my calendars".

For more info: https://support.google.com/calendar/answer/6084659?

To remove the calendar

• In the top right, click Settings.
• Under "settings for my calendars" on the left, click the Contacts/Birthdays calendar
• At the very bottom in the section called 'Remove Calendar' click on the Unsubscribe button.

*Note that you cannot add notifications — for instance by email or SMS — to your Google Calendar birthday calendar. You can get an RSS feed of upcoming birthdays, though.

Therefore, if you would like to get alerts, you can also copy individual birthdays to a personal calendar (open the event and select copy to my calendar).

Source: http://email.about.com/od/googlecalendartips/qt/How-To-Add-Your-Gmail-Contacts-Birthdays-To-Google-Calendar.htm

If you don't see the new calendar please make sure it's set to sync and set to display: https://digibites.zendesk.com/hc/en-us/articles/200299933-How-do-I-select-which-calendars-to-display-