The opportunity costs associated with the provision of informal care are usually estimated based on the reduced potential of the caregiver to partake in paid work (both in terms of whether they are able to undertake paid work, and if so the hours of work undertaken). In addition to the hours of informal care provided, these opportunity costs are also likely determined by the necessity to perform particular informal care tasks at specific moments of the day. The literature, to date, has largely overlooked this dimension of informal care. We used Dutch data from time use diaries which report patterns of informal care throughout the day which enables investigation of when particular activities are undertaken. We found that whereas some tasks must be performed at a relatively fixed time of day, others are shiftable and can be performed at other times or even on different days. Household and organisation activities are more likely to be undertaken by employed caregivers, and seem largely to be shiftable; whereas personal care contains unshiftable activities. This implies additional opportunity costs of providing personal care tasks - we term these "time-bound" opportunity costs. Since the care recipient's need for care may in part relate to unshiftable tasks, we conclude that one should be careful with using care need as an instrument of informal care in labour supply equations.