Yes, using simplier language often makes things simplier. But also often makes communication not so nice. On the other hand once we get our communication (?down) to a simple and strict level, we have to make sure that we are not wrong with our simple words. Are you, Arslan?

technical note:
"have been rounded" is not past tense.
And actually we can use
Present perfect for future... as written in
https://learnenglish.britishcouncil.org/grammar/english-grammar-reference/present-perfect

I suggest to correct the data type from "long" to "int" (to save beginners from their hours of confusion).

Or to at least start with some propper testing:

[Test]
public void Test22() {

  Assert.AreEqual("3101202331", BackWardsPrime.backwardsPrime(3101202303, 3101202333));
    }

[Test]
public void Test23() {

  Assert.AreEqual("10003101202459", BackWardsPrime.backwardsPrime(10003101202303, 10003101202500));
    }