Can a timer scheduled in AWS SWF workflow fail to deliver back the tick?

We are using AWS SWF for our workflows where we need to schedule an activity based on cron expression. We are evaluating the Cron and using WorkflowClock for creating the timer.

Wnated to get answers for the following questions

1. Can workflow clock sometimes fail to deliver the control back to workflow. Does SWF guarantee the timer will definitely go off and schedule the next decision

2. By what deviation can timer delivered be off by original cron, meaning if we start a timer of 3600 seconds, can timer get delayed and go off by let's say after 3700 seconds . Any p100 data for this ?

3. Are these timers on exactly-once or atleast-once delivery model

1. I believe timer delivery is guaranteed.
2. I don't have data, but I think it is delivered within a second unless there is a major issue with the service going on. Note that delivering timer means adding TimerFired event into the history and scheduling a decision task. If a workflow worker is down then actual timer processing can be postponed for a long time.
3. They are delivered to a workflow exactly once.