Time Zone Lookup

iPhemeris uses an international, open source Time Zone database known as TZDATA. You can read more about it here: TZDATA on Wikipedia

• Time Zone lookup requires Internet access (WIFI or a cellular data connection).
• It requires that the: Date, Time and Location are already set.
• Find Time Zone – This button queries the database for the Time Zone in effect for the data and time at the location.
• Local Mean Time – Calculates a Time Zone based purely on Longitude. This is only recommended for researchers and very old dates.

TZDATA is contributed to and corrected by computer scientists all over the world. It was originally created to be an accurate source of Time Zone data for computer clocks which need to know when a specific location switches between regular and Summer or Daylight time DST. Due to the various and politically complex dates when places around the world have adopted and repealed Summer Time, the database can sometimes be off by 1 hour in a specific location. This is happening less and less frequently due to continuous updating and improvement.  Anyone finding errors can report it to TZData by providing 3 sources proving the error, e.g. newspaper articles, etc. TZDATA is now the most accurate historical record of time zone data and clock changes anywhere in the world.

For US locations, errors sometimes occur for dates prior to 1966 (when DST was mandated nationwide). When errors of this type occur, it is typically that the date for the switch between DT and DST is incorrect. This will produce an ascendant difference of approximately 15 degrees. If you are seeing an unfamiliar ascendant or midheaven value, do not assume iPhemeris is wrong, do a Google search for when Daylight/Summer time was adopted at the place of the chart being cast.

Some software uses an older source for historical time zone and daylight savings changes called the ACS Atlas and which is based on The Olsen Database, the predecessor to TZDATA. Unfortunately, the ACS atlas is known to be extremely error prone, so much so that the developers of TZDATA decided they could not use it as their starting point. They analyzed ACS’s accuracy and uncovered too many errors. The developers of iPhemeris have also observed over the years many errors in charts based on ACS. We therefore concluded that the safest approach was not to include it, and we suggest that when a discrepancy arises between TZDATA and a chart cast using ACS, to simply do the research.

Time zones and Daylight / Summer Time are an important subject for Astrologers to be familiar with and it will be well worth your time to study a bit about the subject.