OK, where are we at on this. I heard my name. Deeper thinking takes a little more free time for me to sit down at a computer and try things out instead of hacking away on my phone.
Just quickly scanning through all the posts so far, regarding the date calculations, maybe you already have it figured out…but, largely, as @Darren_Murphy pointed out, if all depends on your definition of a month. Is it a calculation based on a number of days (such as approximately 30 days) or is it based on finding the the same exact day number, but in a different month. For example 2 months prior to today could be represented 60 days ago (Dec. 15th 2020) or 2 months ago (Dec. 13th 2020). I’m guessing based on your situation, you’re more concerned with the actual number of days, so it can be loosely based on the number of days, which I think was covered above.
To save the dates, what I would do is enable user profiles and row owners on that sheet. Then place all of the dates and calculations in that user profile sheet. When they sign in, it will create a new row automatically. Then they can enter their wedding date and everything will be calculated and unique to only them. Row Owners ensures that they only have access to only their own data. Otherwise you could filter each row by signed in user.
Like @Darren_Murphy said, there’s a lot of ways to approach it and it all depends on how you want the app to flow. Other options are a form where they simply enter a date, but it won’t calculate in real time. Once they submit the form, then you can display their row of data and the form sheet would have all of the calculations. They could then edit the date later if they chose. The only downside is there are a lot of extra clicks and you don’t always have real time calculations, unless you have entry components on the details screen.
Also want to mention that I’d be careful with the swipe layout if you aren’t using it for it’s intended purpose of swiping. I recall another post where someone had issues with things not working right because they were using the swipe layout.