Last Updated formula

Hi,
I'm creating a spreadsheet which I have one cell that puts todays date in '=Today()', however I also want to put in a date when the spreadsheet / cells was last updated.

I was wondering is there is a formula that could autopopulate a date depending on the file modification date so that when the file is saved, it updated the cell.

Another variation I'm looking for is if we can identify when a cell was modified (I know this can be done on a shared file). I could then run conditional flags to highlight if the cell had not been updated in the last 7 days.

Please accept my appreciation in advance if you have the knowledge to resolve my strange query.

Thanks & Regards
Mark