How large is the difference in practice?

Rarely more than about an hour. Within a time zone the longitude correction alone can reach roughly half an hour at the edges, and the equation of time adds up to about plus or minus sixteen minutes depending on the date. In most cases the total is small; the point is that it is not zero.

Does the correction change the year or month pillar?

Almost never. Those depend on the solar year and the solar terms, which are astronomical events with their own precise timing. The correction mainly affects the hour period, and therefore the hour character and the element count.

What if I do not know my birth time?

Leave it blank. The Blueprint then calculates with noon and flags the hour period as an approximation, so you can see that the distribution is less precise. Most people fall within one element of their true figure, but we would rather show the approximation than hide it.

Is this what other tools do?

Many skip it entirely and use raw clock time. Some apply the longitude correction but not the equation of time. We apply both and show the numbers, because the whole point of the calculation is that you can check it.

What is True Solar Time?

True Solar Time is the moment the sun actually reaches your local meridian, which is usually not the time on your wall clock. It differs from standard time by two corrections: the longitude correction and the equation of time. Together they rarely exceed an hour — but that is enough to move a birth time into a different two-hour period.

Why the clock and the sun disagree

Time zones are a political convenience. A single zone covers a wide strip of longitude, so the sun crosses the sky at a different moment in every city inside it. If you were born in Los Angeles at 9:00 in the morning, the sun was already well up — but in New York, at the same clock time, it had barely risen. Your birth moment, as the sun saw it, was different in the two cities even though the clocks agreed.

The traditional calendar calculation is sensitive to this. It assigns a two-hour period based on the sun's position, so an uncorrected clock time can put you in the wrong period.

The two corrections

First, the longitude correction: the difference between your city's longitude and the central meridian of its time zone, multiplied by four minutes per degree. A city on the western edge of its zone runs behind the sun; a city on the eastern edge runs ahead.

Second, the equation of time: the seasonal difference between apparent and mean solar time, caused by the Earth's orbit and the tilt of its axis. It swings roughly plus or minus sixteen minutes across the year, and we compute it with the Spencer formula, which is accurate to about a minute.

How we implement it

You give us a date, a clock time and a city. We look up the city's longitude from a table of roughly 120 cities, take its time zone from the IANA database, and derive the correct offset for that date — daylight saving included, because the time zone database already knows when it applies and we do not second-guess it.

Then we add the longitude correction and the equation of time, and we show you the result: the offset in minutes, and the corrected hour we used. If you were born without a recorded time, we use noon and say so.

Longitude correction: (your longitude − your zone's central meridian) × 4 minutes per degree.
Equation of time: Spencer (1971) simplified formula, accurate to about ±1 minute.
Daylight saving is handled by the IANA time zone, never by a hand-written rule.

When it changes the answer

Usually it does not. The two-hour periods of the traditional calendar are wide, and a correction of twenty minutes only matters if it moves you across a boundary. When it does, the hour character changes, and the element distribution shifts with it — which is why the Blueprint prints both the corrected hour and the offset.

Two honest caveats. First, if your birth time was recorded approximately — 'around 2 in the morning' — the correction inherits that uncertainty, and no amount of arithmetic fixes it. Second, historical time zone rules changed in many countries before the 1970s; we use current rules, so very old birth times carry an additional error we cannot remove.

Questions people ask

How large is the difference in practice?: Rarely more than about an hour. Within a time zone the longitude correction alone can reach roughly half an hour at the edges, and the equation of time adds up to about plus or minus sixteen minutes depending on the date. In most cases the total is small; the point is that it is not zero.
Does the correction change the year or month pillar?: Almost never. Those depend on the solar year and the solar terms, which are astronomical events with their own precise timing. The correction mainly affects the hour period, and therefore the hour character and the element count.
What if I do not know my birth time?: Leave it blank. The Blueprint then calculates with noon and flags the hour period as an approximation, so you can see that the distribution is less precise. Most people fall within one element of their true figure, but we would rather show the approximation than hide it.
Is this what other tools do?: Many skip it entirely and use raw clock time. Some apply the longitude correction but not the equation of time. We apply both and show the numbers, because the whole point of the calculation is that you can check it.

Keep reading

Calculate with True Solar TimeFree: enter your birth date, time and city and see the correction applied.
What is the Five Elements?The phase vocabulary the corrected calculation feeds into.
Presence, not predictionWhy we show you a calculation instead of an outcome.

This content is a self-reflection tool based on a traditional Chinese calendar calculation. It is not a fortune telling, divination, or prediction service, and it is not medical, legal, financial, or investment advice.