You might be able to make it work anyway. By changing the due date to the 1st of the month, you will also change the next statement close date. The statement close date will always be 3 days after the preceding statement's due date. Points will not post to your account until the statement in which you met the minimum spends ends.

Example:

Statement ends Dec 4th. The next statement period will be Dec 5 - Jan 4th. Complete minimum spending anytime after Dec 5 and the points will post after that statement ends in Jan. This is assuming you met that spend and it posted to your account within 90 days from account opening.

For what it's worth I applied for two Southwest cards September 19, 2015 and by changing due dates I was able to make those points hit in early 2016. I now have the companion pass until Dec 31, 2017.