Formats does not address this. You have a couple of options.
Use a 1-5, 3-5 nested list and repeat the date in question. So under the US Events (which would be 1-5) you would have two entries that begin with 1776.
Use either emphasis or grouping indicators. In this case, since there is no bold in this section, you could put the World Events in bold. That means your whole timeline would be 1-5, 3-5. The 1776 would be in cell 1 with each event in 3-5 and the World Event enclosed in grouping indicators or enclosed in the bold passage/terminator. When using emphasis, it is imperative that you first check the entire selection and be sure you use something that is not in the print text (in this case, you couldn't use italics because there are italics within the timeline).