Research to date has not addressed the difficulties of coordinating across time zones in global software development. We present a preliminary collaboration model to help us understand the consequences of time separation on coordination costs. The model is for a team composed of dyads and each dyad consists of a task requestor and a task producer who have a sequential workflow dependency. The model is constructed with formulas for: production, coordination, and vulnerability costs for a number of: (1) collaboration modes; (2) time overlap conditions; (3) asynchronous and synchronous communications mechanisms, each of different quality and cost; and (4) production and delay cost rates. We describe the model and evaluate it with regression analysis using randomly generated observations. Our evaluation shows that the model adequately represents time-separated work and that time-separation effects are: (1) different and more complex than distance-separation effects; (2) asymmetric, depending on whether work time overlap between the two actors occurs at the beginning or end of an actor?s day; and (3) dependent on the amount of this overlap.