Code day length is the total duration between your first and last coding activity each day. It is an approximation for length of day worked and helps you think about when your work day starts and ends.
For example, if you begin coding at 10am, the start of your code day will be 10am. During the workday, you may need to attend meetings, collaborate with coworkers, or research something new. If you resume coding at 2pm and stop coding at 4pm, your code day length will be 6hours (10am to 4pm).