About time and sometime/once

Hi there!

I have two question:

1: The other day I asked an English person what time it was. As I didn't hear his answer properly I wanted to make him say again. But instead of pardon, sorry etc., I ask: HOW MANY?
In my mother language the most common way to ask someone to tell the time is "HOW MUCH/HOW MANY IS THE TIME?" Can it be proper in English in any way or is it just obviously wrong?
Because what my confusion comes from is that the person (who was an English) repeated it immediately like there was nothing wrong with the question.

2: Recently I learnt the world: sometime. This is to refer an act that I did or will do but I can't remember (or not decided yet) when exactly. So if I say: "I will do that sometime", I mean that I plan to do that particular thing but I have not decided yet when. (Correct me if I'm wrong! )
Now, up 'till I learnt this world I used "once" to express the same thing.
So my question is: "I did that once" means I did that only one time, but could this mean also, that I definitely did that but either I can't remember or just don't want to say when that was exactly? (Maybe because it's not important.)
And if the answer is yes, can this be used the same way in the future as well?

I hope it doesn't count as a too long question! (It's my first one here.)

Thx in advance!