What clock your birth time is on

How Arcus works out the offset

Arcus works out the offset for every chart from the birth place and date, so you rarely have to set it yourself. The place gives the zone, and the date decides what that zone's clock read at that moment, daylight saving included.

Clocks since 1970 are well recorded, and every program agrees on them. For older dates the shared time zone record that phones and computers use is thin, so Arcus carries its own tables and reads them first. They cover old local and national clocks the shared record merges away, American zone boundaries back to 1919, American daylight saving from the years when each state, county and city decided for itself, and the old clocks of the rest of the world. Before a country adopted standard time, Arcus casts a birth there on the town's own local mean time, or on the capital's clock where the whole nation kept it. When a chart is cast on local mean time, the line under Location in the New chart and Edit chart windows reads Local mean time, with the offset beside it.

Set the offset by hand

You can set the offset yourself when your source puts the birth on a different clock from the one Arcus worked out. Why two programs can disagree on an old birth is on why another program may show a different chart. In the New chart and Edit chart windows, the line under Location shows the coordinates and the offset Arcus worked out. Click Adjust to change them. Write the offset west of Greenwich positive, such as +5:00 for New York or -1:00 for British Summer Time.

Leave the field empty and the offset follows the place, which is how every chart starts. Once you type one, Auto appears beside it and hands the chart back to the place and the date. What you type wins over every table above.

Zones on the charts you import

When you import a spreadsheet, Arcus reads each row's time zone from the file wherever it gives one. A spreadsheet from another program may carry an offset, a zone name, or neither.

• An offset written as 5W or 5E settles itself. Programs disagree about which way a bare number runs, so for a bare number Arcus asks with a control that reads East of Greenwich, New York is -5 or West of Greenwich, New York is 5.
• A full zone name like Europe/London is used as given. An abbreviation like EST is too vague to cast on, so Arcus leaves it alone.
• A row with no coordinates and no offset is held out of the import until you look the place up at the review step.

The rest of the spreadsheet steps are on importing charts from a spreadsheet.