x axis errors in chapter 1 histograms

R 2.15 on OS X
gglpot2 0.9.0
scales 0.2.0

01-Using_R/code/ufo_sightings.R

I wonder if

``` R
scale_x_date(major = "50 years")
```

should be something like

``` R
scale_x_date(breaks = date_breaks("50 years"), labels = date_format("%Y"))
```
