The timeline on the About page is made using the hover for other widget.
Full instructions for hover for other can be found here.
Each tooltip and date is nested into a state button. This is the trigger.
Then the content for each date is the hovered “other” object. These have 0% opacity for the normal state and 100% opacity for the hover state.
Create New Timeline Items
To create new timeline items, simply duplicate one of the existing ones and update the tooltip names for the trigger and hovered object.
Please note that hover for other does not work for touch devices since you cannot have hover effects on touch devices.