A computer system for providing virtualization services may execute computer programs by a virtual processor in a virtual machine. The computer programs may be executed as tasks scheduled for execution at respective points in an apparent time tracked by an apparent-time reference. During execution of the computer programs, the computer system may detect a current point in apparent time at which all tasks scheduled for repeated execution at a given frequency have been executed, or at which the virtual processor is idle. And in response, the computer system may advance the apparent time to a subsequent point with a frequency greater than that with which the apparent time is tracked by the apparent-time reference.