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: an intercept engine that intercepts scheduled executions of portions of computer code; a re-coding engine that modifies original portions of computer code for the activity before the original portions of computer code are executed by the web browsing application, the modifying including inserting code for pre-work operations that are arranged to be executed before respective original portions of computer code are executed and code for post-work operations that are arranged to be executed after respective original portions of computer code are executed for at least some of the original 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 original portions of computer code; a timer that tracks one or more execution durations of the activity for the modified portions of computer code; and an activities log that stores information that describes the one or more execution durations of the activity.