<p>Once considered “wasted”, travel time has increasingly evolved into a time–space locus for activities. In this context, travel time use may influence how people organise daily activities and impact how individuals travel. Yet, travel time use has been investigated in isolation from the broader time-use context, so we remain unaware of how travel-based and out-of-trip activities complement or substitute for one another. This research investigates the relationships between travel-based and out-of-trip activities, taking “activity transfer” (i.e., engaging in travel-based activities to free up time outside the journey) into consideration. To model time use while accounting for such relationships, we calibrate an extended multiple discrete continuous (eMDC) model using revealed-preference data from a sample that represents the adult population living in Greater Melbourne and Geelong (Australia). Our results suggest that travel-based and out-of-trip mandatory and maintenance activities tend to complement each other. By contrast, out-of-trip and travel-based discretionary activities exhibit a substitution relationship. Activity transfer seems to weaken complementarity and strengthen substitution relationships between travel-based and out-of-trip activities. These results suggest that relationships that hold in the broader time-use context also hold within trips, given that travel time accommodates excess demand from high-priority activities and creates a space–time locus for those experiencing leisure-time scarcity. Our findings have implications for transport practice and policy, as they highlight the need for tailored interventions to cater for the activity needs of specific population segments.</p>