Changing date with math?

Hello fellow kirupians!

I have another problem, still with the same calculator. I want to change date with some math…

As you can see on the top tool called “Sparverktyg” at https://www.ekonomikompassen.se/app/ you can add how much money you’re saving, having at the start and how much you can put aside each month and the date to the right shows when you saving will be finished!

So what I want to know is how to change date with math. I have made a formula that may be way hell wrong or possibly working.

The total ammount you’re saving = ss (9000)
Starting money = sk (1000)
Money you can put aside each month = spar (500)
Money left to complete the saving = kvar (8000)

ddmmyyyy = (kvar / spar ((= xmm)) spar * xmm = kvar)

What do you think? And even if it’s right, I still don’t have a clue of what to do…

Anyone???