Is there any sort of date/time stamp for when the record of an individual is modified?
In a future version could there be such a field and we have access to it? That way we could create a group for all individuals whose details have been modified or created in a time period of our choosing, such as in the last day. Then we could use that group to do a proper synchronise with ancestry.
The requested date/time stamp exists. You can see it on the Edit Person screen. It's hidden in plain sight on the lower left hand corner of the screen.
You can make the date/time stamp into a column in People View, and you can sort the column ascending or descending.
You can print the date/time stamp as a column in Custom Reports, and you can sort the column ascending or descending.
You can search for the date/time stamp and color code based on it and make groups based on it. In the Searching or Marking dialog, it's called Date Last Edited.
That's the good news. The bad news is that the date/time stamp can be somewhat unreliable at times. It can sometimes be updated to today's date when in fact nothing for the individual has been changed. It can sometimes fail to be updated to today's date when in fact something has been changed.
The main problem with changing the date/time stamp when nothing has been changed is when you open a note and close it with Ok when nothing in the note has been changed. The design of RM suggests that if you haven't changed a note you should cancel out of it instead of Ok out of it. But I refuse ever to cancel out of a note for fear of losing changes. Until I switched to always exiting notes with Ok, I regularly lost changes to notes by exiting with cancel by accident.
I'm less clear about all the cases where changes are made without updating the date/time stamp. Certainly bulk changes such as making changes to places in the Place List or making changes with Search->Search and Replace do not update the date/time stamp. I would have to do a lot of experimenting to be sure, but I think that changes to sources and media do not update the date/time stamp. That's certainly true if you make the changes from Lists->Source List or Lists->Media Gallery and it's certainly true if you make media changes via any variation of the media tagging process. I'm less sure about making changes to sources or media directly from the Edit Person screen, but I think even there that source and media changes do not always update the date/time stamp.