These methods are not generating any HTML, so they should not be in a view helper. Like you said, they are not needed for controller function, so they shouldn't be in the ApplicationController. So I would put them in your model.
You can put this logic in a module in your lib/ directory. This way if you need the code in different models, then you can just require/include it.
module ChunkyTime
def set_start_time ... def set_end_time ... def get_time_blocks ...
end
class MyModel include ChunkyTime
end
