Is there a way to get the date/time when a acf last modified ?
Do you mean that you want to know when the content of a single field was last modified?
Yes, or the group of fields, actually anything can tells me there is an update, add/remove happened
If you’re using ACF5 and you have local JSON enabled (create a folder named acf-json in your theme folder) the JSON files tell you when they group was modified. Other than than I don’t know of anything built into ACF that will tell you when any changes are made.