We recently were not satisfied with the accuracy of the clock in a conference room. I gave the room a clock which shows accurate time by reusing an old Android tablet. By now, many technical people have several old Android tablets or phones laying around, and one of these can be reused as a clock or digital picture frame.
The main problem is finding a suitable mobile app for the job. There are quite a few clock apps, and several digital picture frame apps which can also show the time. For a digital picture frame, the Floating Image app has quite a few options, including showing from some online sources. However, for our needs the Dock Clock app works better because it will show large numerals (although the size and font can be adjusted).
I downloaded the app from the Google Play Store and tinkered with the configuration to meet the situation. Large numbers on a dark background works best where it was put. It is perched on a simple wire book-holder type of stand — consider using a little Velcro to hold the stand in place and to hold the tablet to the stand.
There now is a FieldGuide Gismodo post with more ideas for old mobile devices.