Live virtual machine (VM) migration technology is a powerful primitive in cloud computing. We introduce a novel "just-in-time (JIT)" live virtual machine migration model and presents a system called Pacer that achieves it. The JIT migration model gives the user a single "knob" -- the ability to specify a desired migration finish time, and Pacer manages the migration to finish as close to the desired finish time as possible. Pacer enables the user to achieve a predictable migration time, which is important for time-sensitive management tasks such as load balancing, scheduled maintenance, disaster preparation, etc. An integral benefit of Pacer is that migration proceeds at as slow a speed as allowed by the desired finish time to incur the minimal necessary performance degradation on the application.