Western astrology can say something useful without your birth time. Saju struggles, because the hour is a full pillar — a quarter of the chart.
What you lose without it. Two of the eight characters, and with them the hour pillar's meaning: output, what you produce, and the later portion of life. You also lose whatever that pillar contributed to the element balance, which can change which element reads as heavy or missing.
What you keep. The other three pillars, including the Day Master, which is fixed by the date. A three-pillar reading is a real reading. It is not a broken four-pillar reading.
Why guessing is worse than omitting. An invented hour adds two characters that are probably wrong, and those characters participate in every relationship the chart describes. A confident wrong answer is less useful than an honest partial one. If you do not know your time, use the "I don't know" option rather than picking something plausible.
The midnight problem. The Saju day begins at 자시, the two-hour block spanning 23:00 to 01:00 — which straddles the calendar midnight. Traditions differ on whether a birth at 23:30 belongs to the current day or the next (야자시 versus 조자시), and the two conventions can produce different day pillars, meaning a different Day Master. Any calculator handling births in that window should be explicit about which convention it uses.
Ours is: the day pillar advances at 23:00, not at civil midnight. A birth at 23:30 on 1 January is read as the 子 hour of 2 January's day pillar, and the hour stem is derived from that next day's stem — the 정자시 convention. Every other minute of the day is identical under either rule; only the 23:00–24:00 window differs. If you were born in that hour and you have seen a different Day Master elsewhere, this is almost certainly why.
Where to find your time. Birth certificates in many countries record it. In Korea it is often on hospital records rather than the family register. Failing that, parents' memory is imperfect but usually correct to within an hour — and an hour is often enough to land in the right two-hour block, though not always.
For entertainment and self-reflection only. Not advice.