Changing date in feed

I recently connected my blog to mailchimp so I could send out feed-to-email mailings every time there is a new post. When I added a new post the mailings did not get sent out. When I review the email that should have been sent, it looks great except that the date is Dec 31, 1969. Mailchimp tells me that the feed date needs to be correct for the mail to be sent out. I do not know how to change it.