我怎样才能知道时间是否过去了？迅速

I'm developing an app where user should able to choose a time slot. i.e 8:00 AM - 8: 30 AM or 2:00 PM - 2: 30 PM. But I want the user to choose only the time slot based on current time. i.e if its 8PM, he shouldn't able to choose slots till 9PM. I need minimum an hour gap. For now I have just hard coded the time slots as shown below and displaying in a picker view.I'm developing an app where user should able to