Twice a year in most of North America, the UK and Ireland, the clocks change by an hour. In spring one hour of the night is skipped. In autumn one hour is lived twice. A birth time written inside either hour cannot be turned into a single moment without one more piece of information, and a birth chart needs a single moment.
The hour that never existed
When clocks spring forward, the minute after 1:59 am is 3:00 am. Nothing in between appears on any clock that night.
| Place | Date | Skipped clock times |
|---|---|---|
| New York | March 14, 2027 | 2:00 to 2:59 am |
| London | March 28, 2027 | 1:00 to 1:59 am |
| New York | April 5, 1987 | 2:00 to 2:59 am |
A certificate that shows 2:30 am on one of those New York dates was still written to some clock. It was written either to the standard time that was ending or to the summer time that was beginning. The two readings are an hour apart.
Take 2:30 am on March 14, 2027 in New York. Read on the standard clock (EST, five hours behind UTC), it is 07:30 UTC, the moment the new clock showed as 3:30 am. Read on the summer clock (EDT, four hours behind UTC), it is 06:30 UTC, the moment the old clock showed as 1:30 am. Each reading is a real moment, but not the same one.
If the certificate gives no clue, you can open both. Pick one, read the chart, then go back and pick the other. The two charts differ only by that hour.
The hour that happened twice
When clocks fall back, the hour after 1:59 am starts again at 1:00 am. Every minute of that hour happens twice, first on summer time and then on standard time.
| Place | Date | Clock time | First pass | Second pass |
|---|---|---|---|---|
| New York | November 7, 2027 | 1:30 am | 05:30 UTC (EDT) | 06:30 UTC (EST) |
| London | October 31, 2027 | 1:30 am | 00:30 UTC (BST) | 01:30 UTC (GMT) |
Both moments are real. The birth was in one of them, and they are exactly one hour apart as instants.
What the calculator does instead of guessing
When a birth time falls in one of these hours, the free birth chart stops before it draws anything and asks.
For a skipped time, the screen says "That clock time never existed in" the city, explains that the certificate "was written either to the time that was ending or to the one that was beginning," and offers two choices: "Written to standard time" and "Written to the new summer time."
For a repeated time, the screen says the time "happened twice that day" and offers "The first pass" and "The second pass."
The chart is drawn only after you pick. The choice is applied to the BaZi pillars as well, so both readings are built from the same instant.
Time zone rules changed over time
The clock rules for a place are not fixed. The dates of daylight saving in the United States changed in 1987 and again in 2007, and some years ran on unusual clocks altogether. The calculator reads the rule that was in force on your birth date from the IANA time zone database.
For a small number of zones whose records before 1970 are known to be uncertain, the result adds a note asking you to check the time on the certificate.
Unusual clocks in February: war time and winter daylight time →
How much an hour moves a chart
The planets barely move in an hour. What moves is the frame they are placed in. In the calculator's own words, "The whole chart turns about one degree every four minutes," and "the Ascendant crosses a full sign in roughly two hours."
So an hour of difference turns the whole chart by about fifteen degrees, and the Ascendant moves with it, faster or slower depending on the sign. With whole-sign houses, which the result screen uses, that can move the rising sign and shift which house each planet sits in.
The Four Pillars use the same moment
The BaZi calculator works from the same corrected instant. An hour can change the hour pillar, and near solar midnight it can change the day pillar too. The Day Master page shows where solar midnight falls for several cities.
Where solar midnight falls, city by city →
Try it
Enter a birth in one of the hours above, for example 2:30 am on March 14, 2027 in New York, and the calculator asks which clock was meant before it draws the chart. Try 1:30 am on November 7, 2027 as well, and pick each pass in turn. The second pass draws the sky one hour later than the first.