To keep things simple you can place controls on your form that are bound to your timestamp field. You can set their visble property to = No.
You will need to type the correct names of your controls in the code. Here I am using the example name of the timestamp. You need to create an event handler.
You can do this by going to one of the controls on your form, while in design view, and double clicking the control.
You want to add code to the event handler for the control taht will be receiveing the update from the user. Double clicking one of these controls will open the Properties window for that control. Click on the event Tab of the Properties window. In the field "After Update" click the elipses (...) button. Then choose "Code Builder". This will create a new event handler where you can place your code. Use the following code
Code:
Me.Field1TS.Value = Now()
With that, if the first control gets updated the other will receive the current date and time, as soon as the user navigates away from the control or to a new record. You need to make sure the fields you create to store the timestamp have the data type Date/Time.