The Hard Part of Birth Chart Calculations Isn't the Zodiac. It's Time.

The most annoying bugs I’ve dealt with while building a birth-chart engine were not about zodiac signs. They were about time. And the deeper I got into it, the more I realized that “birth time” is a much less simple input than it looks on a form. A local datetime isn't enough Take this: 1990-05-15 09:30 It looks precise. But precise where? Without a timezone, it doesn’t identify an instant. So the calculation API I use takes both the local datetime and the IANA timezone: chart = engine . natal ( local_datetime = " 1990-05-15T09:30:00 " , timezone = " Europe/London " , latitude = 51.5074 , longitude =- 0.1278 , ) I prefer an IANA zone like Europe/London over something like UTC+1 . The former describes a real timezone with historical rules. The latter is just an offset. DST makes this more interesting During a daylight-saving fall-back transition, the same local clock time can occur twice. So a value like: 01:30 may correspond to two different UTC instants. That means the input looks exact to the user while still being ambiguous to the calculation engine. There’s a strong temptation to quietly choose one. I don’t like doing that. If the input is genuinely ambiguous, I’d rather make the ambiguity explicit. The same thing happens in the opposite direction during spring transitions. Some local clock times never existed. If the clock jumped directly from 01:59 to 03:00, then: 02:30 isn’t a valid local instant. Again, silently “fixing” it is convenient. But now the software has changed the user’s data. Then there’s the bigger problem: no birth time A lot of people simply don’t know what time they were born. This creates a product decision. You can say: unknown → 12:00 PM and suddenly everything works. You get: Ascendant houses house cusps Midheaven The object looks complete. But none of those values are based on a birth time the person actually supplied. That bothered me enough that I made unknown birth time a first-class state in the engine. chart = engine . natal ( local