Stop columns from resizing unevenly

Try adding this:

.calendar-table { table-layout: fixed; }

In fact, I might add this to the default CSS in the future.

I would also suggest fixing your year jump box by altering this section:

.calendar-date-switcher select {
border: 1px #D6DED5 solid;
margin: 0;
font-size: .9em;
padding: 5px 0;
}