I'm aware of this calculator, USCIS is doing something nice there, which they aren't even required to do by law. Even if OP used this calculator, he would have put Resident Since date + 3 years in the date field, but ignored other messages saying all other criteria should be met such as living in marital union.
This calculator can be improved by asking questions:
- 3 or 5 years rule
- How many days spent outside the US
- If 3 year rule, when started living together
- Any trips over 180 days
But again, this is not a service USCIS is required to provide.
I think I may build one for fun myself one day...