Date of Last Modified for a Cell

I came here looking for a solution to the same objective. From this thread and others I managed to make the perfect excel formula and so wanted to share for anyone else that comes looking.  This solution creates a last modified time stamp that accounts for any change (not just from null value to value) and if the target cell(s) does not fulfil the specification of Last Modified it leaves any existing time stamp as it is without overwriting it to something undesired.  

Solution(s):

=if(Cell(“address”,A1)=Cell(“address”),Now(),C1)

alternatively if wish to get a last modified date when any of a set group of cells is modified, then use the OR function to list all of the logic tests:

=if(OR(Cell(“address”,A1)=Cell(“address”),Cell(“address”,B1)=Cell(“address”)),Now(),C1)

(if anyone knows if a range or array can be used instead I’d be keen to learn your solution too as individually referencing all cells in a large group to be monitored can be lengthy).

Explanations:

So in both of these examples C1 is the cell the formula is being entered into, it references itself and so the iterative calculation needs to be turned on.

Cell(“address”) brings back the address of the last “active” cell for the user - this should encompass the last cell they modified but could also include cells that were “active” but unchanged.

Cell(“address”,A1) brings back the address of the cell you are checking if it has been modified. 

Now() returns timestamp, cell formatting to be used to specify date format & level of accuracy.

Ergo - If the cell(s) I am checking/monitoring is also the last ”active” cell, then update this checker cell with time stamp for now, else leave this checker cell as it was (ie it doesn’t repopulate the checker cell as blank if it is not the most recently modified cell and instead leaves the time stamp from when it last was).

Limitations:-

Activated cells that were unchanged will also be subject to the modified timestamp. 
Am unsure of the impact to cells that update based on formulas/other cell updates. 
Last “active” cell is for the user - not sure of impact when multiple users are editing the same workbook.