photo date incorrect after export from Iphoto on OS

To expand on Matti Haveri's suggestion: There's nothing you can do to keep the original date when exporting pictures in Photos. Matti's suggestions are 3rd party apps we use after the picture has been exported to the Finder.

Another option is to use a 3rd Party app called PowerPhotos that lots of people here use. It costs a bit (maybe $30, but you get a free trial), but it does add lots of flexibility to Photos. For example, it also has an Export command, which leads to this:

You can set "File creation date" to "Date Captured," which is the thing you wanted to do. As you can see, you have a lot more things you can control, as well.

Copy the DateTimeOriginal metadata inside the image (if it exists) to the file's (fragile) creation and modification dates with some 3rd party tool like GraphicConverter, A Better Finder Rename, exiftool etc.