What I do (on Windows OS), is right click on the file and look at properties. (this may be what you’re doing, as well and I’m sure there’s something similar for Macs).
If I look at an old non-modified photo from 2014, I find:
Created: Tuesday, December 10, 2019
Modified: Saturday, April 12, 2014
Accessed: Tuesday, December 10, 2019
This photo was in folders that identified it as being taken on April 12. When I look at a processed photo (one that reflects some cropping and maybe moving an original photo into a new Photoshop file), the Modified date is April 13.
The 2019 dates are when I got my new computer and the file was moved from the old harddrive to the current C drive.
So, I would say, if the modified date is the same month/year as the folder you have the photo in, then it’s fairly likely the day shown is correct as well. Especially if it’s the original photo that was never edited by a piece of software. Although it wouldn’t be a tight reading of iNat’s guidelines, I wouldn’t think being a day or two off would make a difference.
Personally, I do think being off more than a day or two can make a difference. At least where I am, birds can show up for migration in a very tight window of time. If one sees bird species X on April 1st, it would be an aberration. But if one sees bird species X on April 29th, it would be within the known/expected range of time for observing that bird. The same could be true for development of young (hatchlings) or buds on trees or flowers of different plant life.
I’m not sure if that helped any. It’s just something I had to deal with when I wanted to upload old photos here and I’m always eager to offer some solution a person hasn’t thought of. Thankfully, 95% of my old observations were put into folders where I had designated the day/month/year and I had keep detailed notes about where and when I saw birds throughout the year. I knew those OCD tendencies would come in handy someday! I did have to pass over a few photos that I hadn’t keep good enough notes on.