Are there certain times during the day when you can't get tan?

I have heard that there are certain times during the day when you can't get tan from being in the sun, is this true? I've heard after 4pm or before 10am, but this sounds wrong. I don't need an answer talking about the perils of sun exposure, all I want to know is if there are indeed times of the day when the sun is shining yet you cannot get tan. Thanks.