To be clear, we’re referring to the Timeline column type which you can add to a board, although the Timeline view can be used as well.
See Type in Start/End Date in Timeline Column
Monday does not have a hard and fast concept known as a Start Date such as that found in Microsoft Project.
Monday is more flexible. It offers a Date column type which can have multiple uses.
You can add multiple Dates columns into a board and name them Start Date and Finish Date.
You can then use the Start Date as a start date in other boards, views and dashboards.