@M1m1c15, @Woollensock2, or @UnclePeet?

Isn't this interesting?
I just noticed your own username, @Mark12291229.
Let's break it down a little bit. Let's put a space between the letters and numbers.
Mark 12291229
Remove the name, "Mark," and then divide the numbers evenly, there's 8 of them, so separate them into 2 groups of 4.
1229 1229
It looks like the numbers repeat themselves. Let's just remove 4 of those numbers.
1229
Now let's put a space between the two 2's.
12 29
OK great. Now for the punchline.
What's the 12th month and the 29th day?
@December_Twentynine