R help - Dec 2010 - Calculating weekly/bi-monthly average for time series data

Dear all, given a daily time series data, I am able to calculate monthly
average, quarterly average like:
 
library(zoo)
dat <- zooreg(rnorm(500), start=as.Date("2000-01-01"), frequency=1)
mo.ave <- aggregate(dat, as.yearmon(index(dat)), "mean")
head(dat)
head(mo.ave)

However is there any direct way like above to calculate the weekly average,
bi-monthly average?
 
Thanks for your help


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