Drag date bug

[Rajiv Vyas <vyas frys com>]

Here's a bug that I noticed when I use Ctrl and drag a date so that it
would drag in sequence. The problem is that Gnumeric does not recognize
how many days there are in a month. Which means that if I use Ctrl and
drag the the date July 27, than it would drag on until July 35. We know
there is no month in a year with more than 31 days.