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Does your birth time actually matter?
A new sign rises roughly every two hours, so most birth times have room to be wrong. Some do not. This works out which of the two you are, and gives you the answer in minutes.
Your birth details are never sent anywhere. The calculation runs on your own device, and so does the place lookup: the town list is part of the page, so what you type into it is never sent off to be looked up.
Your birth details
1900 to 2099.
As it is written on your record, however rounded.
Pick a town from the list — the check needs its coordinates, so a typed name on its own will not do. If your village is not listed, the nearest town is close enough for a rising sign.
Filled in from the place and date, using the time-zone history in your browser. Correct it if you know the clocks said something else.
What this page is doing
1Three details, kept on your device
Date, time and place. The place list is a gazetteer already in the page, so even the lookup happens on your device.
2The rising sign is computed, then nudged
We work out the sign and degree rising over your birthplace, then move the time a quarter of a minute at a time in both directions until the sign changes.
3You get the window, in minutes
How far out your time can be before the rising sign becomes a different one — and which sign it becomes on each side.
What a comfortable window does not tell you
The rising sign is one rung of a ladder. The sign itself survives half an hour of error in most charts — but the house cusps move with every minute, and the divisional charts a reading leans on change every thirteen to fifteen minutes. A birth time that is safe for your rising sign can still be too rough for the finer layers.
The full ladder is here, measured layer by layer, including the technique that voids itself by its own stated standard.
If your birth time is uncertain or unknown
A window of a few minutes means the time needs to come from a record rather than a memory. If you do not have one,what still works without a birth time is a real list, not a consolation — planetary positions, the Moon's nakshatra on most days, and the period sequence all survive.
Where these numbers come from, and how exact they are
The ascendant is computed by trigonometry from the sidereal time at your birthplace, with the Lahiri ayanamsa. The ayanamsa is a quadratic fit to the Swiss Ephemeris value, matching the convention our chart files use.
Against the Swiss Ephemeris — which is what computes the readings — we measured this over three hundred charts spread across 1900 to 2099. The rising degree came out within about a tenth of an arcminute typically and one and a half arcminutes at worst. Put in the units this page answers in, that is under three seconds of birth time in every case, so it cannot move a window reported in whole minutes. It would matter in a chart file, which is why a chart file is not computed this way.
Two honest limits. The time-zone offset comes from your browser's time-zone database, which is reliable for recent decades and less so for the early twentieth century, when local mean time and short-lived wartime clock changes were common — correct the offset yourself if you know it. And the window is reported rounded down, so the true figure is up to a minute more generous than the one shown, never less.
If it turns out your time matters
A chart file states the ayanamsa, computes every position from the Swiss Ephemeris, and says so where a claim depends on a birth time you are not sure of. See what a file contains.