Hey there, Ryan!
Hmm... well, it looks like we are only offering to go back (or forward) a decade with the datepicker javascript. And unfortunately, when disabling that code, there's no real clue as to how to format that information in the field. We will definitely pass this information along to see what we can iron out for future releases.
However, for now, there are a couple of options that might be helpful. If you want to continue to use the plugin, you could add some "pre-filled" text in that field by adding a predefined value for your list:
How do I set default merge values (like Dear Customer)?: http://eepurl.com/hcDC
So, for example, you could use "mm/dd/yyyy" as the predefined text for that area so that your subscribers know exactly how to format their date field. Here's generally how it will look in the form:
Screenshot -- http://cl.ly/3F2Y0V1B3746180B021Z
Or, if you want to get a little fancier, you can also use a slightly more intensive method, laid out in this post:
http://wordpress.org/support/topic/plugin-mailchimp-list-subscribe-form-request-add-placeholder-text-for-mailchimp-widget?replies=12
The other option, which isn't ideal, would be to forego the plugin entirely and use the embedded form. This can have mixed results depending on what (if any) other plugins are running on the site since it relies on javascript to validate everything inline. However, if you'd like to give it a go, here's the info on how:
How can I add my signup form on my website?: http://eepurl.com/gOGN
And of course, if we can help with anything else, give us a shout. We're happy to help!
Cheers