A time managing device of a computer system including a graphic user interface capable of displaying application windows is disclosed. The time managing device includes an image capturing device, a sight-light detecting unit and a reminding unit. The image capturing device is used for capturing a user image corresponding to a user. The sight-light detecting unit is coupled to the image capturing device and used for analyzing a user sight-light state according to the user image to generate a sight-light detection result. The reminding unit is coupled to the sight-light detecting unit and the graphic user interface, and used for performing a reminder to a predetermined application window displayed on the graphic user interface according to a predetermined time and the sight-light detection result.