I really miss the Batch Change facility in Photos, especially because the Collections format is great for cataloguing most photos, but some are not date related and many of mine had the wrong dates (e.g. scanned oldies of my ancestors).
Editing Time/Date in Photos is problematic. As mentioned elsewhere, the dates change within groups but not outside them. The dates only add time to each, so grouping does not work well. Also, giving several photos the same date works fine in Collections, but only the last one will appear in iOS devices. You need to add a minute or so onto each photo, which you can do in iPhoto.
Would you believe me if I told you there is a 16 minute discrepancy between times on the two apps, so avoid making the first one 00.00.00 as it will then go into the previous day. Start at 12.01.00. and add a minute to each.
What I've done for non-date related photos, is to poodlefake the year into groups for "samples", "odd ones", "portraits", "art" etc as 100, 200, 300... This puts the groups at the start of the Collections view and out of the way of my chronological photos.
But you must do all this in iPhoto, before you migrate, as Photos does not have these Batch change options.
Maybe there is a better way!