Altering date that activiti uses can be done using the class org.activiti.engine.impl.util.ClockUtil.
But you still need to wait until the job-executor has finished processing the timer-job, after changing the current time. Take a look at the method waitForJobExecutorToProcessAllJobs on src.main.java.org.activiti.engine.impl.test.AbstractActivitiTestCase, for a safe way to do this…