Now let's revisit the example rule and circumstances from a Timeline perspective.
Firstly, let's slightly reword the requirement as follows:
rule (reworded): A person is considered to be a lone parent of a minor whenever the person is:
- not married; and
- has a dependent who is younger than 16 years of age.
(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.)
Now let's draw timelines (of type Timeline<Boolean> for when each of the persons is:
- married; and
- has a dependent under 16 years of age.
Figure 1. Timelines for Joe, Mary and James's circumstances
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:
Figure 2. Timelines for Joe, Mary and James's status as a lone parent of a minor
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:
- Joe is a lone parent of a minor from 1st December 1998 to 31th December 2000 inclusive;
- Mary is a lone parent of a minor from 1st May 2004 to 30th June 2006 inclusive; and
- James is never a lone parent of a minor.