Users will be able to edit time & date in Google Photos soon, as a teardown of the APK of the latest version of the app (which is version 2.7 and has actually started hitting devices) reveals references to the capability. The string of code found in the APK that alludes to this functionality for users is listed as "edit_datetime_icon" but is also accompanied by more detail mentioning that there will be an actual icon that users can tap which will bring up the ability to change the time and the date in the photo's EXIF data. While Google Photos is not likely to get this information wrong most of the time, that doesn't mean that it can't ever be inaccurate and that is probably the reason why Google is adding this capability into the app, as they want users' pictures to have the right time and date info.
While the correct time and date may seem like it doesn't matter all that much, and to some users it probably won't, it actually plays a big role in the smarts of Google Photos. The app does a great job of categorizing all your pictures based on various forms of data, and time and date is part of that. For instance, if a user is looking for a particular photo or album of photos and is searching for them using the time and date, it would be more challenging to find them if the time and date were inaccurate. Now that users will have the ability to change this information, that will be an issue that can be corrected if it ever arises.
According to the information provided, it looks like the icon to edit the time and date details will be in the picture details screen, so users would need to tap on this first to find the icon, and from there would be able to edit the time and date as needed. While this option is not yet available and it probably won't be added until at least the next update, just the notion that it's on the way should make plenty of users happy. Once it arrives, it'll be joining other recently added features like the one-tap dynamic filters.