Birth Time, Time Zones and Daylight Saving: Getting the Clock Right

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Enter your birth time exactly as the clocks showed it where you were born. Do not convert it to UTC, and do not take an hour off for summer time. The free birth chart finds the time zone from the coordinates of the birthplace you pick and converts the local time itself, including daylight saving for that date. Converting it first means it gets converted twice, and the chart ends up an hour or more out.

An hour is a lot. Depending on the sign rising, it can move the Ascendant anywhere from about 10° to more than 30° and shift most of the house placements, which is a bigger error than the rounding most people worry about. How accurate your birth time needs to be shows the scale of it.

How the conversion works

When you choose a place from the location suggestions, the calculator gets its latitude and longitude and looks up the time zone for that spot. It then converts your local date and time to Universal Time using the standard time zone database that most software relies on. That database records daylight saving rules for each region over the years, including many wartime changes.

Two examples show what that covers. In June 1943, clocks in Britain ran two hours ahead of Greenwich, and even in winter they stayed one hour ahead. A birth at 12:00 in London is converted to 10:00 UT in June 1943 and 11:00 UT in January 1943. In New York, January 1943 fell under year-round wartime daylight time, so 12:00 local becomes 16:00 UT rather than the usual 17:00. You do not need to know any of this to use the chart; it is applied for you.

Because the zone comes from coordinates, pick the actual town of birth. Near a time zone border, the nearest big city can be on the wrong side of it.

Clock-change hours

Twice a year in places with daylight saving, the clock does something awkward.

Spring forward. One hour is skipped. In Amsterdam on 31 March 2024, clocks jumped from 02:00 straight to 03:00, so 02:30 never happened. If you enter a time inside that gap, the free chart shows an error saying the time does not exist rather than quietly shifting it.

Fall back. One hour happens twice. In Amsterdam on 27 October 2024, the clocks went from 03:00 back to 02:00, so 02:30 occurred once in summer time and again an hour later in winter time. Enter a time inside that hour and the free chart reports it as ambiguous instead of picking one for you. In New York, under the rules used in recent years, the same thing happens between 01:00 and 02:00 on the first Sunday of November.

If your recorded time falls in a repeated hour, look for anything on the record that says which one it was. Without that, there are two possible charts an hour apart. The free chart cannot currently take a choice between them, but you can get a sense of what is at stake by comparing the last minute before the repeated hour with the first minute after it. Neither of those is your chart. They simply bracket it.

Older births need more care

The time zone database is very good for recent decades and less complete further back. Its maintainers say plainly that it is built around clocks agreeing since 1970 and that earlier entries cover only part of how clocks actually behaved; the IANA notes on the scope and accuracy of the tz database go into detail. In some regions, places that share clock rules today are merged into one zone, so a town's own earlier history (local mean time, an unusual offset, a different start date for summer time) may be simplified.

So if you were born before 1970, and especially before the Second World War, it is worth checking what the clocks were set to at that date and place. Local histories, almanacs and national time services are good places to look. The free chart always uses the database's offset and has no field for a manual correction. If you find a mismatch, note it and treat the Rising sign and houses as provisional, the same way you would with a rounded time.

Other slips worth checking

• 12-hour times. "12:30" on a record can mean just after midnight or just after midday. A midnight birth may also be written under either date.
• Times written in another zone. A birth on a ship or aircraft, or one recorded by an institution working to another zone, may carry a time from a different clock. If the record says which zone it used, convert to the local time of the birthplace before entering it.
• Copied times. A time that has passed through several family members may have picked up or lost an hour along the way. Go back to the earliest source you can find; how to find your birth time lists where to look.

If the time cannot be pinned down

Sometimes a clock-change hour, an old record and a vague memory all stack up, and the honest answer is that the time is uncertain by an hour or more. In that case it is better to read the chart as if the time were unknown than to trust a chart built on a guess. What your chart can show without a birth time explains what stays reliable. The calculator's wider choices, such as the Tropical zodiac and Placidus houses, are set out on the methodology page.

Related articles

• How Accurate Does Your Birth Time Need to Be?
• How to Find Your Birth Time
• Birth Chart Calculator Guide
• Why Birth Time Matters