pretty_dates crashes for intervals greater than one year

[garrettgman]

From David A.

pretty_dates fails to compute the intervals whenever it has to extend beyond intervals of 1 year. Consider the example:

pretty_dates(c(as.Date("1993-12-05"), as.Date("2007-12-01")), 14)
As expected, it returns "1993-01-01 UTC", "1994-01-01 UTC", etc.

Now change the third argument to 7 instead of 14.
pretty_dates(c(as.Date("1993-12-05"), as.Date("2007-12-01")), 7)
One would expect "1993-01-01 UTC", "1995-01-01 UTC", etc., but instead it crashes.