It should probably be a simple Boolean field because:
- Marketo can't calculate and store intervals between Dates internally (you'd have to call a webhook, and then you might as well just pass the Purchase Date and Created At to the 'hook and get back a true/false result)
- While Marketo can subtract score tokens from score fields, which is a form of Integer arithmetic, it can't do this with 2 lead tokens (so even if you had the dates converted to Integer epoch times, you couldn't get the difference between them without, again, calling a webhook, and if you're doing that there's no need to convert in the first place)
On the other hand, if you mean an SFDC formula field, then using a Date + Formula does make sense.
But Marketo doesn't have built-in functionality like SFDC's, so you'll end up calling an external service, and that service can operate directly on (static) Date fields and return a Boolean.
P.S. In case it isn't clear to anyone who's lurking, Marketo absolutely can calculate the diff between dates in a Velocity script (e.g. to output different email content based on date math). But the result of that calculation is ephemeral. It can't be written back to a lead record,* therefore can't be used to qualify leads for Smart Campaigns or viewed in reports.
* OK, there is one way, but the method is so exceptional and undocumented that it's best to pretend it doesn't exist.
Hmmm - Thanks so much Sandy. It actually looks a little bit more complicated that we need at the moment so I think we will do an extraction and then cross check with our client purchase date....