Technically this concept should not be that difficult to grasp. A period of time 30 minutes long. BUT NO!
time period has a life of its own. It appears to mean any time after now. Common uses include:
I'll meet you at (insert meeting point here) in half an hour
I'm just popping out for half an hour
I'll hand in that urgent piece of work in half an hour
See also back in 5 minutes