I had the same problem, when I had a forum which was updated from phpBB v.2 with Birthday mod by Niels installed.
After forum updatem all fields in database which was coming from Birthday mod became part of new phpBB3 installation, and show valid birthday date on profile automatically. I found it very cool, cause I didn't need to ask my users to update their birthday dates. But...
After installing UBL by lefty I found that forum doesn't show today's birthday on index page, but they were before in upcoming, as Gular found. Firstly I thought that this is lefty's mod problem but it wasn't. I took two databases (one with v2 and Birthday mod, and second with updated phpbb3). In some way database information about birthday date was "converted" wrong, altough for the first glance two databases looks the same. Don't know what is the problem, but I found a cure.
Users, who updated their date on phpBB3 have their birthday date shown on index correctly. Those who didn't do that will have an information in upcomming birthdays, but it will not show when their birtday day comes.
Gular, if you have the situation as I described above, you have to ask your old users to update their profile. They don't need to change anythig but go on UCP, when birthday date is shown and click a button.
Concluding, I think that the problem isn't with lefty's mod, but it helped me to find a bug.