Working time reduction (WTR) is a promising policy to enhance well-being in rich countries and an important topic in discourses on a new social vision. Numerous small-scale WTR trials are either underway or planned in various contexts. Properly measuring changes in working time is necessary to evaluate these trials, but challenges abound. Traditional definitions and measurement methods may not work for fragmented, creative, and location-independent jobs. The primary aim of this article is to review relevant work-time definitions and data-collection methods, discuss their complexities, and summarize the implications for WTR research. We reviewed 243 articles and categorize indicators currently used in the literature by relying on 45 methodological studies to identify the main challenges and potential solutions. We conclude that the most dominant definitions and methods, notably usual weekly hours measured by worker surveys, are losing relevance and credibility in many contexts. With the rise of indicators focused on actual hours and measured by time diaries, work grids, interviews, automatic measurements, and time-sampling, we foresee the emergence of non-comparable, job-specific work-time indicators. We propose a new time-sampling approach to deal with some of the least measurable jobs.