Might be useful here, too (posted this in the rpietila wall observer thread):

This version of the formula directly works on Wolfram Alpha (copy/paste):

10^(-2.869800 + 0.003012 * ((number of days since 2009 Jan 03)/days))

or just follow this link:

http://www.wolframalpha.com/input/?i=10%5E%28-2.869800+%2B+0.003012+*+%28%28number+of+days+since+2009+Jan+03%29%2Fdays%29%29