Exported video creation date wrong

I'm using Photos to export video taken using iPhone. The date the video was taken was (for example) 8th May 2016, but when exported using the 'export unmodified original' method, the date of the video changes to todays (or a very recent date).

When I two-finger tap and 'get info' the creation date is 15th September 2016. When I use a program such as Media-info to inspect the file, it shows the real creation date of 8th May 2016. This tells me the real date is accessible there somewhere but Finder isn't using it.

I'm sure I haven't had this problem in the past. Maybe it's because the video has been stored in iCloud only for a period? (both MBP and iPhone rather full). The date shown in Photos is the correct 8th May 2016 one also btw.

Anyway, this is problematic for trying to get the files chronological in iMovie as i'm combining iPhone video with camcorder video, so really need creation dates right. Any ideas?