Clock to use when giving presentations
Digital Clock does almost everything you ask. Ignore the rubbish picture from the page, it is a full-screen clock with white text on black and it's free.

To configure it as you need, launch the app, then open the settings (tap the screen to open the options then tap Settings) then set:


Clock->Screen timeout to unchecked.
Clock->Orientation to Landscape.
Time &amp; Date->Show date to unchecked.


You can also adjust the time format and colour if you wish. The time is about half the screen height, but I can easily read it on my phone from more than 20ft away so that may not be a problem.