Emerald Time is a little iPhone™ application that uses Network Time Protocol (NTP) servers on the Internet to obtain a more accurate time than is typically available from the iPhone's internal clock, which can be off by several seconds or more.
The middle right window shows the offset (in seconds) between the internal clock and the network time; positive means the network time is ahead of the internal clock.
The four items inside the cloud represent some of the NTP servers that have been contacted. While Emerald Time is actually in communication with a server it shows that server's time. After it disconnects only the difference between that server's clock and the iPhone's internal clock is shown. The best value (not necessarily the smallest one) is shown in green.
The red/yellow/green lights above the time show the progress of the server connections. When Emerald Time has enough good data the green light is on. The other lights appear only while data is being collected.
There are seven hot spots that you can tap. Tapping on AM/PM, time zone or hours:minutes will cycle between 12- and 24-hour time and between your local time zone and UTC ("GMT").
Tapping the seconds will cycle between seconds with tenths, just seconds, and nothing.
For the truly obsessed (and for the convenience of the developers) tapping anywhere on the cloud brings up a table of server statistics as shown to the right: "n" is the number of good packets and bad packets received; "offset" is the difference between that server's clock and the internal clock; "σ" is the standard deviation; "rtt" is the average round-trip time to that server (in milliseconds). The colors indicate the state of the server connection. Tap again to return to the cloud.
Tapping any of the lights makes Emerald Time immediately contact the NTP servers again to recompute a new offset. That burns up some network packets and makes a nice light show, especially with the server info table showing.
The "i" brings up the built-in version of this documentation.
Emerald Time has an option to disable the iPhone's normal auto-lock so that the iPhone does not sleep while Emerald Time is running (see the Settings app at the bottom of the first page). This will drain your battery in just a few hours; best to keep it on the charger.
Emerald Time periodically goes out to the Internet to get data from the NTP servers so beware of data roaming charges. If the network is unavailable, it will try repeatedly with longer and longer intervals between attempts. You can always force another attempt by pressing the lights.
Don't be surprised if the offset changes by more than the standard deviation. The iPhone frequently updates its internal clock from the cell carrier. Emerald Time sees that, contacts the NTP servers again and calculates the new offset. The displayed time should not change but the new offset may be quite different from the previous one. If your friends' iPhones are also running Emerald Time their offsets may be different but the displayed times should all be the same!
Emerald Time primarily uses servers from the NTP Pool Project but it also includes Apple's default NTP server and one directly connected to the official German standard atomic clock ("Atomuhr CS2").