“I don’t know my birth time” is the most common reason a chart comes back incomplete. It is worth understanding why that one detail matters as much as it does.
The sun barely moves in a day. The horizon does a full lap.
Your sun sign only needs a birth date because the sun spends roughly a month in each sign — the hour you were born makes no difference to it.
The horizon works completely differently. The Earth completes a full rotation every twenty-four hours, which means the point rising on the eastern horizon sweeps through the entire zodiac in a single day. That point is your rising sign, and it changes roughly every two hours.
Born at 7am instead of 9am on the same date, in the same city, and your rising sign is likely different.
What actually depends on the time
Three parts of the chart are time-sensitive, in order of sensitivity.
The rising sign is the most sensitive of all, changing roughly every two hours. This is the placement most likely to be wrong if your recorded time is off by more than an hour or so.
The houses are calculated directly from the horizon position, so an inaccurate time throws off which house every planet falls in — even though the planets’ signs stay correct.
The moon is less sensitive but not immune. It changes sign roughly every two and a half days, so it is only at risk near a sign boundary, but when a birth falls close to that boundary, the exact hour can matter.
Your sun sign and the signs of the slower outer planets are unaffected by time of day. Those are safe even with an approximate time.
Where to find your actual birth time
A birth certificate is the most reliable source and usually has it printed. Hospital records are the next best option if the certificate does not.
Failing both, a parent’s memory is worth trying — even an approximate time, if it is confident and specific, gets the houses roughly right. “Sometime in the afternoon” is not enough for the rising sign; “around 3pm, I remember because visiting hours had just started” usually is.
If you genuinely cannot find it
A chart without a birth time can still be generated — most calculators will default to a fixed hour, commonly noon — but treat anything involving houses or the rising sign as unreliable in that case. The planetary signs themselves remain accurate.
Getting the mechanics right matters, but it is only the starting point — what you do with an accurate chart afterward is where most people find the actual value.
Once you do have a time, even an approximate one, run it through a proper birth chart calculator rather than guessing at the rising sign from a generic table. The calculation is precise enough that ten minutes’ difference in your recorded time can occasionally matter, and it is not something worth estimating by hand.