Only I like the idea that Clark, a child who actually wasn't born on planet Earth, also wasn't born on a day that exists in the year 1966.
It makes some odd sort of sense. He's technically untraceable in all aspects.
Of course, even if February 29th existed in Krypton in 1966, for his Earth birthday you'd have to trace his birthday back on an Earth day... which would make it February 28th again.
And I totally just wrote the above mostly needless explanation to justify posting the following, really short, comment:
This would explain why he never seems to age. Only having a birthday once every 4 years.
