In some cases you might see timecode displayed as unexpected values in the IN and OUT area of the preview window. By default, this timecode is generated from the time-of-day of the shot. For example, if the IN marker says 09:14:22:00, that is the time of day when the shutter button was pressed. This is used for syncing sources from multiple cameras and/or to keep track of shots if you are keeping a shot log.
If you want the number to start at 00:00:00:00, open the application Preferences (GoPro Studio/Preferences...) and deselect "USE TIME OF DAY FOR TIMECODE". This setting does not apply to clips which have already been converted.
Note that if you use video from certain sources such as the Canon 5D which generates a timecode track, the numbers will still be the timecode from the source.