Clark gives his birthday as February 29th in Never On Sunday:
CLARK: Interesting trick.
SUNDAY: No trick. I only made use of what was already in his mind. (he regards Clark) You're a... Pisces, right?
CLARK: Good guess. February 29th.
SUNDAY: Nineteen sixty... five?
CLARK: Close. Sixty six.
Although I vaguely recall that citing a flurry of 'that doesn't make any sense' posts shortly after the episode aired.
And given the mysteriously floating date of the Kents' wedding anniversary, who knows when his birthday was by the time we got to AKA Superman.
LabRat