This has always been an issue.
If the default time zone is EST and my profile is EST why do Dec 1 Birthdays show up when it's still technically Nov 30?
It's as if the internal clock for the site is on a different time zone.
Is it running off of your server clock?
What is your timezone set for in your PHP settings? I have noticed an issue in the past where the settings in PHP for timezone needed to be adjusted.
This sounds like you are using a plugin for birthdays as well which would need to be moved to the third party discussions if so. I can move it. Just need to know if you are using a birthdays plugin.
In checking the time, when posting here it is showing correctly for my timezone which is EST. Just one test of the time.