Skimming the article doesn't make it clear whether they've addressed this, but, if you want to add units of time of non-constant size, then you cannot just use time-stamps. For example, `(today + 1 year) - today` is longer than `(today + 2 years) - (today + 1 year)` [0], so that one can't think of them as just `(today.unix + seconds_in_year).human` and `(today.unix + 2*seconds_in_year).human`.

[0] Assuming that today + 1 year is defined to be 2017-01-21, and today + 2 years is defined to be 2018-01-21; and, if not, then one faces other problems with intuition.