Pennsylvania celebrates its 223rd birthday this Sunday.

And you probably forgot to send a card.

It's not too late to commemorate the date. After all, the Commonwealth became the second state on Dec. 12, 1787.

But Sunday is auspicious for an additional reason, according to an electrical engineering professor at the University of Portland.

The date, which can be written 12122010, is unusual enough to fascinate numerologists, the type of folks who used to look for mystical meanings in numbers - but are now play and derive a curious sense of delight over their properties.

Aziz S. Inan points out that 12122010 is an extremely rare date number, because it's reverse, 01022121, is a perfect square number. In other words, it's the product of 1011 x 1011.

"There are only three such dates in the 21st century," Inan said. "The other two reverse perfect squares dates in this century are Dec. 14, 2030 (12142030 where its reverse, 03024121, is the product of 1739 x 1739) and Oct. 20, 2062 (10202062 where 26020201 is the product of 5101 x 5101)."

Got that?

(For what it's worth there were only two such dates in the 19th century and four in the 20th)

But wait, there's more!

"What makes 12122010 even more special is that when it's written without its rightmost zero-digit as 1212201, it is also a perfect square (1212201 is the product of 1101 times 1101)," Inan said. "So 12122010 is an extremely unique one-of-a kind date number . . ."

There's a peck of other peculiarities wrapped up in 12122010. For other numerical wonders see Inan's website at http://faculty.up.edu/ainan/ .

So, at this point, we're sure we've convinced you that Sunday is indeed a date worth celebrating.

But good luck finding a greeting card commemorating the date at the local CVS or Walgreens.

Contact staff writer Sam Wood at 215-854-2796 or samwood@phillynews.com.