In buddhism there is no such thing as a birthday and at the same time there is.
Here the solution.
In Buddhism there's two realities.
There's the perceived world, were you live and dance and eat and eat your birthdaycake.
Then there's the "world as it is", where everything is at it is, and not necessarily as you think it is.
Take such a thing as time.
It's a human invention, maybe conceived to simplify things, but nonetheless, a human invention.
In the "real world" there may be some thing like time, but i don't know.
What i do "know" is that in my "perceived world" there's time.
And yesterday i had an birthday.
Birthday means that in some distant time, i had an birth, and that day that was some years ago, came again yesterday.
According to Buddhism there is no yesterday (well there is but it is now, and that means yesterday is today, but don't separate between hot and cold).
All time is "now" (bad wording, but langauage is limiting...).
And if all time is "now", what about "yesterday" and "tomorrow"?
Is today yesterday tomorrow?
But not as you think...
Thats time, what about birth?
Buddhist doesn't believe in an separate soul, so there is no "separate being being born".
There's plentiful of Buddhist texts pointing this out, most famous is maybe "the Heart Sutra's" "theres no birth, no death""
And if there's no separate being being born, what about birthdays?
Well, there's no separate birth.
No "timeline", so no days.
But we're constantly reborn (buddhists believe in rebirth).
And all time is "now".
So, Happy birthday!