In ancient Hebrew thought - did they even have minutes or seconds?
Did they divide such shorts amounts of time, the same way the Romans did?
I think half-hour (first of all, it says half-hour not 30 minutes) is meant to express a general short period, maybe even less than a day - either way, an abstract period, unspecified, undefined. Perhaps unknowable.
KJV Rev. 8:1 "And when he had opened the seventh seal there was silence in heaven about the space of half an hour" uses the phrase "os - hemiorion" in Greek, which means "about, around, near - hour, period" - so there is definetely an element of obscurity.