Birth dates aren't just birth dates for us. When you enter a date in the tree, we take that information and do a lot with it in order to provide you with different features like Calendar Alerts, Statistics, etc. In order to provide these features, there must be a standard date format that our system works on.
A full date with day, month and year is required in order for us to calculate the information required for these features.
You can always add a custom event in the Events tab. Custom events will show up on your Upcoming Events.