How copy "Modified date" to "Created date"?

Added:

Photos does not support to change the file attributes, only the capture date. Photos is dealing with photos and not files. To change file attributes try tools like A Better Finder.

http://www.publicspace.net/ABetterFinderAttributes/

Where are you seeing the "Date Created" in Photos? The Info panel in Photos is only showing the capture date embedded in the IPTC and EXIF tags of the original image file.

The file creation date you are seeing in the Finder, if you access the originals using the Finder, may change, if your library is synced with iCloud. All the original image files in my iCloud Photo Library on my MacBook Pro Retina are showing a creation date sometime in July, when I downloaded the library from iCloud.

I see the 2015 date both in Photos under See Info, as well as in Finder.

Then the image files do not have an embedded capture tag. Are they perhaps scanned photos? You can correct the date that Photos is using by selecting the images in Photos and using the command "Image > Adjust Date and Time". Photos is using the File creation date only, if no capture date has been embedded by the camera.