Here is a an example of a working plugin that shows users with upcoming birthdays. It consists of:

• Main page widget - shows all site users with upcoming birthdays within 7 days if any.
• Dashboard widget - shows member's friends with upcoming birthdays within 7 days if any.
• Profile widget - shows member's upcoming birthday status within 7 days if available.
• Separate page with the list of users with upcoming birthdays withing 7 days if any.

You can view the above pieces of code and also download the plugin from the Oxwall Store for general plugin creation guidelines.