Previously, when adjusting the date of a photo it offered the option to check a box with the the option to Modify the Original date. But this box and options isn't there.
Photos never had this option. Are you perhaps referring to iPhoto or Aperture?
Photos is a very different application and has been designed to work well with a Photos Library in iCloud. Access to the originals (for example - reveal original in Finder) or batch modifications to the originals have been completely omitted from Photos, probably, because it would be very hard to implement it, with the originals in iCloud. The modification would have to sync to all devices that are syncing with iCloud Photo Library. and it would no longer be possible to adjust the Date and Time, if we are not connected to the internet.
When I need to write changes to the originals I either apply them before importing them to Photos, export and reimport them. But before reimporting you have to delete the previous version from the library completely, including "recently Deleted". And the deletion has to have been synced to iCloud, if you are using iCloud Photos. Otherwise Photos will just continue to use the previously imported original.
