For groovy's `TimeDuration`, it's annoying that there's not an equivalent in guava - it's discussed at [1], but can't see anything in guava 14 at a quick look. Rather than depend on the groovy class, I'd prefer us just to use `Long rollupPeriodMillis`, I think.

[1] https://code.google.com/p/guava-libraries/issues/detail?id=631