plyr - R resetting a cumsum to zero at the start of each year

Question

I have a dataframe with a bunch of donations data. I take the data and arrange it in time order from oldest to most recent gifts. Next I add a column containing a cumulative sum of the gifts over time. The data has multiple years of data and I was looking for a good way to reset the cumsum to 0 at the start of each year (the year starts and ends July 1st for fiscal purposes).

This is how it currently is:

id        date          giftamt      cumsum()
005       01-05-2001     20.00        20.00
007       06-05-2001     25.00        45.00
009       12-05-2001     20.00        65.00
012       02-05-2002     30.00        95.00
015       08-05-2002     50.00       145.00
025       12-05-2002     25.00       170.00
...          ...          ...         ...

this is how I would like it to look:

id        date          giftamt      cumsum()
005       01-05-2001     20.00        20.00
007       06-05-2001     25.00        45.00
009       12-05-2001     20.00        20.00
012       02-05-2002     30.00        50.00
015       08-05-2002     50.00        50.00
025       12-05-2002     25.00        75.00
...          ...          ...          ...

Any suggestions?

UPDATE:

Here's the code that finally worked courtesy of Seb :

#tweak for changing the calendar year to fiscal year
df$year <- as.numeric(format(as.Date(df$giftdate), format="%Y"))
df$month <- as.numeric(format(as.Date(df$giftdate), format="%m"))
df$year <- ifelse(df$month<=6, df$year, df$year+1)

#cum-summing :)
library(plyr)
finalDf <- ddply(df, .(year), summarize, cumsum(as.numeric(as.character(giftamt))))

Answer 1

i would try it this way (df being the dataframe):

#tweak for changing the calendar year to fiscal year
df$year <- format(as.Date(df$date), format="%Y")
df$month <- format(as.Date(df$date), format="%m")
df$year <- ifelse(df$month<=6, year, year+1)

#cum-summing :)
library(plyr)
ddply(df, .(year), summarize, cumsum(giftamt))