Convert __DATE__-style string to sortable string
C, 137 184 184 140 120 106 103 characters

Replaced the month name lookup with a magic formula.
The formula (m[1]*4388^m[2]*7)%252 is ascending for month names.
Changed it to nicely return 0, at no cost.
It no longer prints a number. Instead it prints a string, which should sort right.
Implemented supercat's %*s idea, which inserts more spaces for earlier months, along with a function that's descending for month names - (m[1]*29^m[2]+405)%49.

#include&lt;stdio.h&gt;
int main(void){
    char*m=__DATE__"%*.6s\n"+1;
    return!printf(m+6,(*m*29^m[1]+405)%49,m);
}


I thought single digit days are represented as Jan_1_2012 (_ being a space), when in fact it's Jan__1_2012 (extra space). This complicated things, so my previous versions were more complicated:

#include&lt;stdio.h&gt;
int main(void){
    char*m=__DATE__+1,*t=m+m[4]/16;
    return!printf("%s%3d%s\n",t+3,(*m*4388^m[1]*7)%252,t);
}