From a data frame with timestamped rows (strptime results), what is the best method for aggregating statistics for intervals?

Intervals could be an hour, a day, etc.

I've found the aggregate function, but that doesn't help with assigning each row to an interval. I'm planning on adding a column to the data frame that denotes interval and using that with aggregate, but if there's a better solution it'd be great to hear it.

Thanks for any pointers!