There is a large number of different digital clocks on the market. Knowing how to set a particular make and model can be a real problem.
The following links to videos and manuals may be helpful.
Reviews of clocks can be found here.
Clock Setting Videos
Setting (mode 21)
DGT Easy Plus
English Chess Company Timer (Leap)
Saitek Competition Pro
Saitek Pro III
Example Setting 1
Example Setting 2