For example, is there a semantic difference between the two following sentences?

>Lapis descendit ab A ad B unâ horâ.

vs.

>Lapis descendit ab A ad B una hora.

[This site][1], e.g., says

>The distinction between short and long vowels was grammatically significant, cf. **hora** hour *vs.* **horâ** at (this) hour, now

Is this true?

  [1]: http://www.orbilat.com/Languages/Latin/Grammar/Latin-Pronunciation-Syllable-Accent.html