If you are satisfied with a more or less exact solution, you can write:
=A1+(30*D1/D2)
where:
- A1 is your starting date (in date format), such as: 2014-03-31
- D1 is your total amount of debt, in this example: 20000
- D2 is your monthly sum of debt payed, in this example: 200
The formula simply counts how many months you need to clear the debt (D1/D2, which gives 100), and then multiples it by the average days of a month (30). Then it adds this amount of days to the starting date. Actually this example gives the result: 2022-06-17 as the final clearance of the debt.
As I said it's not fully exact as it counts 30 days as an average month and it does not count loop years, but I think that it can be used for your purpose.
I hope it helps.
UPDATE:
You'll get a little closer result to the exact value, if you use this:
=A1+(365*quotient(D1/D2,12))+(30*MOD(D1/D2,12))
The QUOTIENT function will calculate how many years are needed (in the example D1/D2 divided by 12). We multiply this value with 365 (the number of days in a year).
Then the MOD function calculates how many more months are needed after the years, which is the modulo of "D1/D2 divided by 12".
This function will give 2022-07-27 as a result for this example.