In the $:/ControlPanel, "Appearance", "Toolbars", "Page Toolbar", there is a checkbox for "timestamps".
Select that checkbox to add the "timestamps" (stopwatch) icon to the sidebar buttons.
Once added to the sidebar, you can then toggle the stopwatch to pause/resume updates to timestamps.
note: Be sure to change the setting BEFORE you edit a tiddler. Once you are in "edit mode", toggling the setting doesn't apply to that tiddler.
-e