There's not much to document here. It just returns a class based on the given date and current date (Late - date has past, soon - within 5 days, on-time - more than 5 days).
Expression Engine 1.7
{exp:rel_ddate due_date="{custom_entry_date format="%d %F %Y"}"}
<ul>
<li class="{exp:rel_ddate due_date="{custom_entry_date format="%d %F %Y"}"}">
Sample to-do
</ul>
</ul>
<ul>
<li class="soon">
Sample to-do
</ul>
</ul>