Would be nice to have a `get_last_dn_time` to get the last descending node

[adybbroe]

Code Sample, a minimal, complete, and verifiable piece of code

# Your code here

Problem description

Would be nice to have a get_last_dn_time to get the last descending node similar to the existing get_last_an_time method.

Expected Output

Actual Result, Traceback if applicable

Versions of Python, package at hand and relevant dependencies

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