12345...82931 (51-digits)

This number is a prime.

                    1 2345678910 1112131415 1617181920 2122232425 2627282931

The concatenated dates of the month without 30 is prime. Interesting that if you insert the number 30, you find that the new composite has a prime divisor of 29, the last date in a leap year! Proved prime by APRT-CLE. [Haga]