http://robots.thoughtbot.com/post/12890232727/how-to-find-a-time-format-for-rubys-strftime
a Tue Thu Sat
b Nov Nov Dec
c Tue Nov 1 00:00:00 2011 Thu Nov 17 00:14:27 2011 Sat Dec 31 23:59:59 2011
d 01 17 31
e 1 17 31
f %f %f %f
g 11 11 11