The NOW() function will return the current date and time in a cell and of course you can call this function and enter it into a cell whenever data is changed, but the big problem with this approach is that it is not automatic and the date and time will change when the spreadsheet is recalculated unless you cut and copy the date and time and paste it back as text or some fixed value. Not a lot of help!
The solution lies with using a Visual Basic routine to insert this information for you and preserve the last date and time the data was changed. The exact details are far too complex to clearly lay out here, but a Google search for "Excel recording date and time of data change" will reveal several sites from which you may cut and paste the necessary code to suit your application.
The Visual Basic editor is part and parcel of Excel so you should have no problem following the instructions and getting one of these routines to work for your application.