Automatic date update in a cell when another cell's value changes (as calculated by a formula)

Question

I have a formula in C2, say =A2+B2. Whenever C2 changes value (actual value, not formula) I want to have the present date and time updated in D2.

I have

Answer 1

The simplest way is to add =IF(B3="","Not Allocated",Now()) and change the format of the column to the required date and time format. But here if the B column is edited, the date and time of the respective column that needs the update will gets updated automatically for all the columns since not checking the old value. But if it is fine to get current Time this can be easily used.