My questions is closely related to the one asked here: [Pull Return from first business day of the month from XTS object using R](https://stackoverflow.com/questions/10994496/pull-return-from-first-business-day-of-the-month-from-xts-object-using-r).

Instead of extracting the first day of each month, I want to extract, say the 10th data point of each month. How can I do this?