How it Works
Every day the moment of solar noon travels from east to west around the world. On the east side of your time zone it is solar noon before 12:00 AM standard time, and on the west side of your time zone it is solar noon after 12:00 AM standard time. The 'middle' of your time zone is where mean solar noon and 12:00 AM standard time occur at once. This is the middle time-wise; it is not always at the geographic center.
On the Earthdial you adjust the outer time zone lens so that 12:00 AM is either before or after solar noon, reflecting where you live in your time zone. This is done by first finding the difference between your own longitude and the longitude of the 'middle' of your time zone. This difference is your 'Longitude Offset'. Then set the outer time zone lens before or after solar noon by that number of degrees.
In the example above, Providence, RI is to the east of the middle of the Eastern Time Zone, so solar noon is before 12:00 AM. The longitude of the middle of the Eastern Time Zone is 75 West (5 hours x 15 degrees per hour). Providence is to the east at 71.3 West. The Longitude Offset is 75 - 71.3 = 3.7 degrees to the east. Mean solar noon is 15 minutes before 12:00 AM.
The close-up below shows the scale indicating Longitude Offset degrees on the time zone lens. The black line with the double-ended cross arrow aligns with the white dot on the support post at the ''zero offset' point. From there rotate the lens counterclockwise (east) for east-of-middle locations and clockwise (west) for west-of-middle locations.