If you are emphasizing the need for a specific amount of time to pass, using ‘timer’ is more precise than ‘alarm’. Put the eggs into the boiling water and set the timer for three minutes. I set the alarm for 8:50 to remind me that I have an italki class at 9. The paint isn’t dry enough to add a second coat. I set the timer for two hours an hour ago, so it should go off in an hour. Or ‘in an hour from now.’

Setting an alarm for two hours doesn't really make sense. Setting an alarm for two hours from now is fine. You want to be woken up in two hours or reminded to do something.