Translation:Are you going to leave around twelve?
I'm a native Spanish speaker, and this is not used at all here where I live (Peru), I couldn't even figure out its meaning (to me it sounded like he wanted to say 'past twelve' or after this time), we use 'alrededor de las doce', which is basically the same as in English (around).
Prepositions and prepositional phrases are often difficult to match one-for-one between languages, and "sobre" is no different. Here's one way to think about how it is used in this drill:
The preposition "sobre" is commonly used to mean "on" in English, but "on" in English is also commonly used to mean "on a topic/subject." From there, it's only a small step to substitute "on" with "about" to say "about a topic/subject." Thus, "sobre" often translates into English as either "on" or "about." Interestingly, the sense of "about" meaning "approximately" in English is also shared with "sobre" in Spanish. So, synonyms of "approximately" like "about," "around," "roughly," etc. can all be translated with "sobre."
The idea here is that 12 refers to a time of day and not a date. When speaking about time it's always "las" + number, because it's short for "las horas." I said "always," but that's a lie. It wouldn't make sense to use the plural for "one hour" (i.e., one o'clock), so they don't. But other than that, it's always "las" + number.
If you did want to say "the 12th," I think it would be "el doce," because Spanish doesn't use ordinal numbers for dates except for "the first."
That's because "las doce" is not necessarily noon or midday. It could refer to midnight. For midday, you would expect "mediodía," "las doce de la mañana," or something similar. Obviously with the right context, you could use "las doce" as a reference to midday. Absent any context, however, 12:00 is just 12:00.