1. A computing device, comprising: one or more processors; a web browsing application: a rendering engine that generates a presentation of a web page for display to a user; an input handler that detects a user interaction with the presentation of the web page, the user interaction indicating an intent to invoke performance of an activity by the computing device; and a scripting engine that executes one or more scripts in web code for the web page; and an activity tracker, comprising: a re-coding engine that modifies portions of computer code for the activity before the portions are executed by the web browsing application, the modifying including inserting code for pre-work operations and code for post-work operations for at least some of the portions of computer code, wherein the re-coding engine provides the scripting engine with the modified portions of computer code to be executed rather than the corresponding unmodified portions of computer code; a timer that tracks one or more execution durations of the activity; and an activities log that stores information that describes the one or more execution durations of the activity.