Everybody hopes the answer is roughly. The answer is that the planets forgive an estimate and the angles absolutely do not.
Why the clock matters at all
The planets move slowly enough that an hour barely changes where they are. The Earth, meanwhile, turns once every twenty-four hours, which works out at roughly one degree of angle every four minutes.
Since astrocartography is entirely about the angles — what was overhead, what was rising — the Earth's rotation is the variable your whole map hangs on. An hour of uncertainty is fifteen degrees, and fifteen degrees is the difference between a line through Madrid and a line through the Atlantic.
The lines that tolerate a guess
MC and IC lines are vertical and depend only on longitude, so an error shifts them sideways in a way that is at least predictable. Fifteen minutes out puts them about four degrees out, and they are still broadly telling you something.
Anything derived from the planets themselves rather than the angles is more forgiving still. If all you want is a rough sense of which hemisphere your chart likes, an estimate will not embarrass you.
The lines that need you to be right
Rising and setting lines are curves, and curves move in ways that are much harder to correct for by eye — a modest error near the equator becomes an enormous one at high latitude. Ascendant and Descendant readings from a guessed time should be treated as a rumour rather than a finding.
This is exactly why we would rather you enter *time unknown* than a plausible-sounding invention. A wrong time produces a confident map, and a confident wrong map is worse than an honest gap.
If nobody wrote it down
Try the long-form birth certificate rather than the short one, which is where it usually hides. Depending on where you were born, hospital records or a state vital records office may hold it. Failing that, a parent's memory of *before lunch* is worth more than nothing and less than you would like.
There is also rectification, where an astrologer works backwards from known events to estimate a time. Take it for what it is: an educated reconstruction with a person's judgement inside it, not a recovered fact.