How is a logic bomb different from a time bomb?

The Time Bomb is a subclass of a logic bomb which were designed by programmers who get fired in an act of spite or a cheap shot to get rehired since they can 'trigger' the bomb to go off (which is why it's called a time bomb). A Logic bomb is a like a back door which allows hackers to implant viruses which either preven a system from accessing the main Operating system or expose low security areas.