The core task-spooler formula installs a ts binary, which conflicts with the ts ("timestamp") binary in the moreutils formula. This formula renames Task Spooler's ts to tsp, like Debian does.
brew install smammy/task-spooler-as-tsp/task-spooler-as-tsp
Or brew tap smammy/task-spooler-as-tsp and then brew install task-spooler-as-tsp.
brew help, man brew or check Homebrew's documentation.