[R] Define a variable on a non-standard year interval (Water Years)

[Sam Albers]

Hello,

I am trying to define a different interval for a "year". In hydrology,
a "water year" is defined as the period between October 1st and
September 30 of the following year. I was wondering how I might do
this in R. Say I have a data.frame like the following and I want to
extract a variable with the water year specs as defined above:

df<-data.frame(Date=seq(as.Date("2000/10/1"), as.Date("2003/9/30"), "days"))

## Extract the normal year
df$year <- factor(format(as.Date(df$Date), "%Y"))

So the question is how might I define a variable that extends from
October 1st to September 30th rather than the normal January 1st to
December 31st?

Thanks in advance!

Sam