Segmentation by date

Hi Phil, 
You can use a smart list filter "Data Value was Changed", select the name of the Opt-In field you want to look for, add the constraint "New Value" and select "true." Check out the screenshot below--i used "Unsubscribed" because i don't have an Opt in field, but it's the same concept.
Have fun!
Elizabeth

Hi Phil, unless you've been date stamping when people opt in (which is actually a very good practice), you will only be able to see data value changes for the past 90 days in Marketo. This would mean that you could only currently go back to the end of July... anything prior to that has been archived (removed) from your view in Marketo. Do you have field tracking in your CRM for this field, perhaps?

Thanks for the insight guys... I'm looking into if we have field tracking and date stamping (which I'm 90% sure we do)

What I'm yeilding when I use the "Data Value Changed" filter I get the number of leads who have opted out for that month not the number of opted out leads. Make sense? Any ideas on how to get this number.

you take the total number of opted-out leads today, then subtract how many people unsubscribed between now and July.

Going forward, it would be good to export reports, for example via Report Subscriptions. Then you can save the state of your database at a particular moment in time.