IPhone countdown apps that don't include days that you specify?

So I'm a senior in high school, and graduation is approaching fast. We have our senior countdown in the hallway already. I've been using an app called "countdown" for all of my countdown stuff, but it's not very accurate if it includes weekends and days we have off of school. I've looked at some other countdown apps that have an "exclude weekends" option, but as far as I know, we have two days off during two weeks before the end of school, which those "exclude weekends" option doesn't cover. Does anyone know of a good iPhone app that could give me an accurate countdown, without those days off included in the countdown, and still has the hours, minutes, and seconds counting down? It would be much appreciated if someone could tell me about a good one if they know one (preferably free, but if it's not, then that's fine. As long as I have my accurate countdown.) Please let me know if you find any or have a good one that you use!

Added (1). Also, I'd just like to add that I wouldn't be using this app for just a graduation countdown, but for all my countdowns, so something like a school countdown app won't work. I just would like an app that eliminates counting specific days that I choose, such as if there's a 3 day weekend, which most apps that have the "exclude weekends" option do not have.

SkipCount would be a good option! It is a countdown with the ability to exclude custom days. It does cost 99 cents though.