I want to create a date calculator website, based on a spreadsheet I created. Where do I start?

First, I am not looking for someone to create a website for me! Just want to get that out of the way.

I created a spreadsheet for work, on google sheets, that I use to calculate deadlines for projects. Our deadlines are set by the government and are very specific, so the spreadsheet has been amazingly helpful (essentially, it calculates 60 days out, taking into account weekends and our specific holidays, along with some other variables).

The problem is, I am reaching the limits of what I can do with a spreadsheet and would like to create a website that I and other employees can access, that would work similar to timeanddate.com, using my specific criteria.

I have some (basic) experience with HTML and CSS, and have been looking for an excuse to learn java script or some other language, and figure this would be the perfect excuse.

I was hoping someone could point me in the right direction of where to start. I.e., what languages and such would be involved and what I would need to do to get this accomplished.