The Emerald Observatory app for iPad from Emerald Sequoia LLC is a clock and so much more. In addition to the 12-hr and 24-hr analog time, you can see a heliocentric orrery (display of planets in orbit around the sun) and their rise/set times and altitude and azimuth. You can determine the beginning and ending of civil, nautical and astronomical twilight, the “golden hour” when the sun is 15 degrees above the horizon and the light is generally best for outdoor photography, the equation of time, leap years, and a time zone indicator. The inner subdials show UTC, sidereal, and solar time. There’s also a photographic representation of the lunar phase and apparent size, and a geochron-type display of the earth’s daytime and night areas. This information can be displayed for the current location or for any entered location and for the current time (as synchronized via NTP) or any time from 4000 BCE through 2800 CE. You can set the program to be always on for a beautiful clock display. The Emerald Observatory app is $0.99 from iTunes.