If Calendar timers are created programatically it is possible to use the javax.ejb.ScheduleExpression and set start() end() as a Date.
If the timer is persistent and the database is Oracle (here 12c ojdbc7 driver) a server restart will show the following Exception is:

13:55:00,450 WARN [org.jboss.as.ejb3.timer] (ServerService Thread Pool – 63) WFLYEJB0161: Failed to reinstate timer 'ejb31-timer.ejb31-timer.SimpleScheduleSingletonTimerBean' (id=bd2e59a0-bdf6-43a1-9241-fd3a0b3a72da) from its persistent state: java.sql.SQLException: Invalid conversion requested
at oracle.jdbc.driver.CharCommonAccessor.getTimestamp(CharCommonAccessor.java:420)
at oracle.jdbc.driver.T4CVarcharAccessor.getTimestamp(T4CVarcharAccessor.java:644)
at oracle.jdbc.driver.GeneratedStatement.getTimestamp(GeneratedStatement.java:360)
at oracle.jdbc.driver.GeneratedScrollableResultSet.getTimestamp(GeneratedScrollableResultSet.java:1008)
at org.jboss.jca.adapters.jdbc.WrappedResultSet.getTimestamp(WrappedResultSet.java:2140)
at org.jboss.as.ejb3.timerservice.persistence.database.DatabaseTimerPersistence.timerFromResult(DatabaseTimerPersistence.java:535)
at org.jboss.as.ejb3.timerservice.persistence.database.DatabaseTimerPersistence.loadActiveTimers(DatabaseTimerPersistence.java:452)
at org.jboss.as.ejb3.timerservice.TimerServiceImpl.getActivePersistentTimers(TimerServiceImpl.java:953)
at org.jboss.as.ejb3.timerservice.TimerServiceImpl.restoreTimers(TimerServiceImpl.java:693)
at org.jboss.as.ejb3.timerservice.TimerServiceImpl.activate(TimerServiceImpl.java:225)
at org.jboss.as.ejb3.component.EJBComponent.start(EJBComponent.java:588)
at org.jboss.as.ejb3.component.singleton.SingletonComponent.start(SingletonComponent.java:133)
at org.jboss.as.ee.component.ComponentStartService$1.run(ComponentStartService.java:54)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:748)
at org.jboss.threads.JBossThread.run(JBossThread.java:320)
Caused by: java.lang.IllegalArgumentException: Timestamp format must be yyyy-mm-dd hh:mm:ss[.fffffffff]
at java.sql.Timestamp.valueOf(Timestamp.java:237)
at oracle.jdbc.driver.CharCommonAccessor.getTimestamp(CharCommonAccessor.java:415)
... 18 more

As a result the timer seems created with a new id and the entry within the timer-table is duplicated.