Shweser Volume 1, Practice Exam 2 AM, Q30

If I’m asked to give the trailing PEG ratio at the end of 2008, why am I taking the EPS from the 2008 Income Statement and not 2007. I understand that since Trailing P/E x Net profit margin = P/S, E in both numbers has to be the same, but I’m just wondering why you take E from 2008 when you calculate trailing P/E.