Why does my reminders app constantly go off?

I wasn't sure where to put this discussion, but since it happens on my MacBook Pro and my iPhone, I put it here. Here's my question: Why does my reminders app CONSTANTLY go off? I have a couple set everyday, and it continuously goes off. I check it off each day, but then at random times (before I have completed certain reminders), it goes off. For example, I have one set for 4:00PM every day. It goes off at 4:00, as it's supposed to. Then at 4:21 it goes off again. Then again at 4:39, and then 4:41. Etc. It's very irritating. And then after checking it off, it beeps for the next day, before it's even 8:00PM! It's very irritating. I've checked settings and edit options for specific reminders..... help! T

Thanks.