date: avoid "X years, 12 months" in relative dates
When relative dates are more than about a year ago, we start writing them as "Y years, M months". At the point where we calculate Y and M, we have the time delta specified as a number of days. We calculate these integers as:
Y = days / 365 M = (days % 365 + 15) / 30
This rounds days in the latter half of a month up to the nearest month, so that day 16 is "1 month" (or day 381 is "1 year, 1 month").
We don't round the year at all, though, meaning we can end up with "1 year, 12 months", which is silly; it should just be "2 years".