Automated renewal emails

Not sure if this is the best solution, but you could trigger the campaign off of the date field being changed, then wait 6 months, send email, wait 1 month, send email, wait 1 month, send email. etc.

This of course would require you to also have a method to REMOVE the person from the flow if they had renewed.

A more robust solution likely exists but this is something to get started with

This also assuming that the initial date is updated when a person renews their contract (to begin the campaign) 

The best way to do this is with a filter that has a "in future" operator similar to the "in past" operator or can utilize date math.  Unfortunately, that does not exist.  There is an Idea for this Date Math - please vote for it.

I'd create a field in SFDC that calculates the date to send the first renewal email and sync this field into Marketo.  From there you could run a morning batch program on every lead with a value matching today to begin sending emails until they do something indicating a success.

It seems like you will be able to accomplish this using the new "Wait Until (+/-) Days" feature--part of the March release:  
https://community.marketo.com/MarketoArticle?id=kA050000000L98K


I haven't actually tested this yet, but I'm trying to accomplish the same thing.  The first step is going to be create a custom date field that would contain Expire Date.  You'll probably run into the same question I did on how you need to format the field:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Pa4mAAC

Thanks for all the comments and suggestions! I'm looking forward to seeing the "Wait Until..." token -- sounds like it might have potential.