Official comment
We currently don't have this feature in our Ontraport Pages. We do have plans to include it at a future time, but I don't have an ETA for it at this time.
There are some custom coded and integration partners solutions that set the day dynamically for the countdown timer, but, as of yet, the countdown timer does not allow for 15 minute increments.
That being said, we may have certified consultants who have done this. You can ask in our Facebook User Community also at https://www.facebook.com/groups/ONTRAPORTusercommunity/