Hey Glen,
You're outputting HH:mm:ss.
- That's not a valid time for calculations.
- You need Unix (or Epoch) Time.
It's a common mistake.
See this:
KM Dates & Times: Notes on Anchors, Intervals, and Translations
Search for:
%Calculate%TIME(2015, 8, 23, 18, 44, 0)%
-Chris