It's an issue with the TTS (text to speech). Unfortunately this is something only staff can look into, though it has been reported to them.
However, feel free to send a bug report: https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-
Your usage would be correct if you added "minutes"; "It is twenty minutes before seven."
I find it disrespectful and immature that you have felt the need to hurl insult at the language you have chosen to learn and to disparage its speakers. You have got to understand that certain ways of expressing things in one language may not always make the most sense to some speakers of another language. Even then you do not have to scream it from the rooftops. And chances are you will grow used to whatever it is that you now find "stupid" should you persevere and actually start speaking the language, which it sounds like you may not. I personally would not invest my time and energy in learning a language I was not fond of or didn't have appreciation for.
Dear fakhirkan, I get your point, and yes, you are right. There is no need to express the negativity anywhere, let alone in the classroom. I was trying to say that in my mother tongue we also have the same way of saying things, but almost no one uses it, and I don't like it there also, but that doesn't mean I don't like my language. I didn't try to be disrespectful, It just seems to me unnecessary to complicate things, if they can be plain and simple. And from another comments which came from native Dutch speaker, I see that this can be said in more simple way, and it is in practice, too. My reaction was, indeed written with wrong choice of words. I like Dutch language, but, yes, it can be over complicated in some cases.
Indonesian uses this style (adopted from Dutch).
Tien = sepuluh Over = lebih/lewat Half = setengah Zes = enam Zeven = tujuh Viertig = empat puluh Uur = jam
It is six forty/It is twenty to seven.
Jam enam lewat/lebih empat puluh.
Jam setengah tujuh lewat/lebih sepuluh (menit).
Indeed, in English this would be a bizarre way of forming the time, both because of the adding to the half rather than taking away from the whole, and also using the term 'half seven', which if it were to ever be used in (Australian) English, would most likely be interpreted as 7:30.
I thought French was bad, with soixante-dixneuf and quatre vingt seize! Irish does one better with dá fhichid is a cúig déag for 55 ("Two twenties and five-teen"). But Dutch takes the first prize, for a combination of addition and subtraction. Except for Romans, of course...