It's pretty simple math, really... you can just get the info from the Ibox...
Oct 9th, 2008 to April 21, 2009 = 6.5 months (194 days / 30 days/month)
OS on Oct 9th = 110,905,871
OS on April 21 = 263,877,032
263,877,032 - 110,905,871 = 152,971,161
152,971,161 / 6.5 = 23,534,025 shares diluted per month on average... so he actually rounded down...
EDIT: I assumed Shorts is a male... my apologies if this assumption was incorrect