[Bug 908680] Fix typo in graph cutoff.

JS's `.getDay()` returns 0 through 6 to represent the days of the week. `.getDate()` return 1 through 31 to represent the days of the month. I used the wrong one.

r?

Also, I would like to push this change out today (even though it is Friday) because it is a small changed, and this fixes a regression that went to production yesterday.