The reference time that is the same everywhere, with no zones and no daylight saving.
Astronomical calculations are done in Universal Time, because it is the only unambiguous one. But nobody is born «at 04:00 UT»: you are born at five in the morning in Bucharest. That is why SideraMea always shows both times side by side — and writes only one when they coincide, as in London in winter.
From local time to UT
Converting a birth time takes three pieces of information: the clock time, the place, and the time-zone rules in force there on that day. Those rules are not fixed. Britain has changed its clocks in different ways over the last century, including a few years in which it stayed an hour ahead of Greenwich all year round, and elsewhere whole time zones have moved.
SideraMea works out the time zone from the coordinates of your birthplace and applies the historical rules of the international time-zone database, the same one most computers and phones rely on. So someone born in London at 10:00 on 15 July 1985 was born at 09:00 UT, because British Summer Time was in force; on 15 January that year, the same clock time was 10:00 UT.
How it relates to GMT and UTC
In everyday speech UT, GMT and UTC are used interchangeably, and for a birth chart they are. Strictly speaking, GMT is the historical name for Greenwich Mean Time, UT is its modern astronomical form, tied to the Earth's rotation, and UTC is the civil time kept by atomic clocks, held within a second of UT by the occasional leap second.
A few tenths of a second change nothing in a chart: in that time the Ascendant moves by a tiny fraction of a degree. What really matters lies elsewhere: that the local time you start from is right, and that the correct time zone has been applied to it.
Frequently asked questions
How do I find out which time zone applied when I was born?
Usually you do not have to work it out yourself: astrology software derives the time zone and any daylight saving from the place and date of birth, using the rules in force at the time. What is worth checking is that the time you enter comes from the birth certificate rather than family memory, because that is where most errors start.
I was born the night the clocks changed: which time do I use?
When the clocks go back in autumn, one hour happens twice: in the UK, 01:30 occurs twice, an hour apart in UT. The local time alone cannot tell you which one is yours. If the birth record does not say, it is worth calculating both versions and comparing them: the Ascendant and the houses are what change most.
Why does my birth chart show a different time from when I was born?
Because astronomical calculation works in Universal Time, which ignores time zones and daylight saving. Someone born in Paris or Rome will usually see an hour less in winter and two less in summer; someone born in London in summer, one hour less. It is not a mistake and your birth time has not changed: it is the same instant, written on a different clock.
See it in your own chart
Calculating your birth chart is free: wheel, planets, houses, aspects and a first reading. All you need is your date, time and place of birth.