Calculate downtime while some time ranges are excluded

Hello XLent, thank you very much, and sorry for the late reply.

The formula does seem to work wonderfully, but would you mind explaining how it works exactly? I admit I am not very savvy with Excel.

EDIT: The formula gives an error when start time is close to midnight and end time is before 5AM apparently?
I have attached the Excel file updated with the formula, and marked in yellow where it gives the error; as you can imagine, the formula should return 01:00:00 (1 hour from 23h to 00h00, the 3 hours between 00h00 and 03h00 are not counted).

All best,