The Android system has a property called NEXT_ALARM_FORMATTED. The value of this property is displayed by the lock screen. This is normally the day and time of the next alarm (set by the Alarm Clock application) but can be set to any string as long as the application that does it has the WRITE_SETTINGS permission.
However, that string gets reset every time an alarm is set or rings and the lock screen only updates when it's started (i.e. when the device is locked) so it's a real PITA to keep it up to date.