Recommend a one hour "getting started wtih iMovie" class (free) at your local Apple Store.

But basically iMovie does create a new file that is created on that date. Not sure if that is a major hold up though.

My iPhone videos appear in Photos on my Macbook. Some times I want to edit them and save them back to Photos. I right click which gives me QuickTime as an option, but it doesn't open. I see from searching that iMovie will do it (and will do more anyway). I can start a new movie project and open the video from Photos but can't see how to save it back, except by exporting a file and re-importing that file into Photos. That changes the date so I go back in and edit the date to what it was. Maybe that's it. I don't know iMovie much -- maybe what I need to do is learn iMovie better first, in which case direction to a tutorial or book would be helpful. Thanks.