def at(time)
            ms = Scheduler.parse_at(time) - @time_diff
            event = OneShot.new(self, ms)
            event.progress &Proc.new if block_given?
            schedule(event)
            event
        end
def at(time)
            ms = Scheduler.parse_at(time) - @time_diff
            event = OneShot.new(self, ms)
            event.progress &Proc.new if block_given?
            schedule(event)
            event
        end
Create a one off event that occurs at a particular date and time

 @param time [String, Time] a representation of a date and time that can be parsed
 @param callback [Proc] a block or method to execute when the event triggers
 @return [::UV::OneShot]