repetitive task timer maintenance

What happens to a timer task when the engine is down for maintenance?

I have a timer task that is set to execute every 30 mins. In the case where I bring down the server for maintenance for more than 3 hours, what will happen to the tasks that were supposed to run in that period? Will all of them get executed one after the other, or will they go parallely or only the next task would be executed and the time for execution of subsequent tasks will be calculated from the time this task is executed?

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