analyze subsample of dataframe

Hi there,

I'm dealing with a pretty big dataset (~22,000 entries) with numerous
entries for every day over a period of several years.  I have a column
"judy" (for Julian Day) with 0 beginning on Jan. 1st of every new year (I
want to compare tendencies between years).  However, in order to control for
a leap year (2004), I simply need to subtract 1 from every judy value for
the year 2004 (without altering judy values from other years!).  What is the
most effective way of going about this?  Can I integrally subset within a
function of some form?

Merci
Tyler