I am not 100% sure whether this feature works correctly but the way I understood it was as follows:
Dynamic Damage - The building gets protected x amount of minutes (I think the value you need to enter is in seconds) after the last clan member logs off. Once a member logs in, they can be raided at any time.
Raid Time - overwrites the dynamic damage, during the raid window you can attack bases regardless whether they have members online or not.
Basically, if you activate them both, you have 24/7 raid time but outside of the raid window, you have offline protection.