Now let's revisit the example rule and circumstances from a Timeline perspective.
Firstly, let's slightly reword the requirement as follows:
(We have removed the phrases "On any given date" and "on that date", and used "whenever" instead. This rewording is subtle, but can be key to help making the mental switch from thinking about points in time, to thinking about data which changes over time.)
From these timelines of Joe, Mary and James's circumstances, let's draw new timelines to derive how their lone-parent-of-a-minor statuses change over time:
Note that we can immediately read how the lone-parent-of-a-minor status of each person changes, without having to guess at interesting dates: