AutoTools Convert HH:mm to seconds & add time

I'm brand new to Tasker and AutoTools, so sorry if I describe things wrong or seem clueless - I am. I'm using the GUI for AutoTools.

I have %Sunset get set to the time of sunset each day and is displayed as HH:mm, i.e. 17:30. This works. I would like to create another variable, %Sunset60, to be used as a trigger for other Tasks and equal 60 minutes after sunset.

I am unable to figure out how to convert %Sunset into seconds so I can manipulate it with math (add 3600 for 60 minutes). Here's what I have:

AutoTools > Time > Time Span

Convert Time Span: %Sunset
Use Seconds: True
Output Variable: set

%settotalseconds
Total Seconds

then as the next Action I have:
Variable Set %Sunsecs1 to %settotalseconds

But the output is either 0 or 0.001 or something. I would expect the correct answer to be 63,000 for 17*3600 + 30*60.

If I could get that working I would then do Variable Set %Sunset60 to %settotalseconds + 3600. But I can't get the conversion of Sunset to seconds working so that part isn't relevant yet.