Many freelancers and workers track how they use their time on a daily basis. There are plenty of online tools and apps that let you do that easily. Post Worktime Logger for WordPress is a simple plugin that allows you to track how long you have worked on a post.
This simple plugin also has a widget for displaying your data to your visitors. You get a box next to the post editor to pause or reset the timer. The plugin is designed to update only when you have actually worked on a specific post. You can find this project on GitHub.