Guides
The time on your birth certificate is not the time of the sky
The time on your birth certificate is local time, and a country's local time changes over the years. This is where most charts go wrong.
Two clocks that rarely agree
Calculating a chart needs the instant in universal time. What you have is a local time: whatever the clock on that hospital wall said. Converting between them looks trivial and is where most people are lost.
The reason is that a place's local time is not a fixed property. It depends on the offset that country applied on that date, and on whether daylight saving was in force that day.
Spain is the perfect example
By longitude the Iberian peninsula sits in the Greenwich zone, the same as Portugal and the United Kingdom. Since 1940 Spain has kept Central European Time, one hour ahead. That gap is not a dinner-table curiosity: it is a full hour applied to every Spanish chart of the last century.
Daylight saving has come and gone on top of it. It was applied irregularly for decades, disappeared in some periods and returned under different rules until European harmonisation. A July date in the nineteen-sixties and a July date in the eighties do not convert the same way.
An hour of error moves the ascendant fifteen degrees
The ascendant advances roughly a degree every four minutes, so an hour of offset shifts it about fifteen degrees. That is usually enough to move it into another sign, and it drags all twelve house cusps with it.
Nobody looks at a printed birth chart and notices that the ascendant is off. The error is silent, which is why it is worth resolving properly rather than trusting that someone will spot it.
How we resolve it
First the city name becomes real latitude and longitude. Then we look up the IANA time zone in force at that exact point on that exact date, with its daylight saving if there was any, and convert to universal time with it.
If the geocoder cannot find the city, the order does not continue. We would rather be unable to sell you a chart than sell you one placed somewhere else.
What gets written down
The certificate that ships with the order records the resolved coordinates and the time zone applied, along with the house system and the zodiac. It is the part that lets the piece be audited without taking our word for anything.
If the zone is not the one you expected, write to us: remaking it beats hanging a chart cast from the wrong hour.
How do I know whether daylight saving applied to my time?
You do not have to. It is resolved from the date and the place, and the result is written on the certificate so you can check it.
What if I was born exactly at the clock change?
The rule in force that night in that country is applied. If the case is genuinely ambiguous, write to us before it goes to production and we will look at it with you.
What if I was born in a country that has changed offset?
The offset in force that day at that point is used, not today's. That is precisely the case the IANA database exists to answer.