The Birthdays add-on gives you the opportunity to mark the site members who are celebrating their birthday today, or have celebrated their birthday recently, or will celebrate soon.
A special icon (balloon by default) is displayed next to the main photos of the birthday boys and girls. The system will also send a birthday greeting by email.
Administration panel > Modules > Birthdays (/admin/birthdays/settings/) offers you a set of parameters to manage the feature:
enable/disable the feature
send birthday emails to users automatically
number of days to display the balloon icon before and after the birthday
Please note the max possible number of days for this setting. Why only 14? It has to do with February that only has 28 days = 2×14 (14 before and 14 after the birthday).