Every minute counts: The added-unit effect in time perception

People encounter and evaluate temporal information every day, for example when evaluating product delivery terms or choosing between online courses. Using data from an online course aggregator platform and nine pre-registered experiments, we show that describing time with a larger and a smaller unit (e.g., “by 8 PM on June 30”, “2 hours and 59 minutes”) rather than with a larger unit alone (e.g., “by the end of day on June 30”, “3 hours”) increases perceived duration. We refer to this phenomenon as the added-unit effect. The added-unit effect emerges because added smaller units change people’s mental representation of time, making them more likely to construe time as a collection of more numerous, smaller units. We document the effect across multiple unit combinations (e.g., dates + hours, hours + minutes, minutes + seconds) and show its implications for online course interest, evaluations of product efficacy, temporal goal setting, and time–money trade-offs. This research contributes to a more nuanced understanding of how number and unit information shape people’s judgments and adds to extant work on time perception.