Partial refactoring of https://github.com/redmine/redmine/blob/master/app/controllers/timelog_controller.rb#L104. That whole controller would need a serious amount of work to be whipped into shape, but this is a start.

redmine_timelog_controller_create.rb

class TimeEntriesController < ApplicationController
  before_action :set_project, :set_issue

  def create
    @time_entry = container.time_entries.build time_entry_params.merge(user: User.current)

    if @time_entry.save
      respond_to do |format|
        format.html { redirect_back_or_default created_time_entry_url, notice: l(:notice_successful_create) }
        format.api  { render :show, status: :created, location: @time_entry }
      end
    else
      respond_to do |format|
        format.html { render :new }
        format.api  { render_validation_errors @time_entry }
      end
    end
  end

  private
    def container
      @issue || @project
    end

    def created_time_entry_url
      if params[:continue]
        polymorphic_url [ @project, @issue, TimeEntry.new ],
          time_entry_id: @time_entry.id, back_url: params[:back_url]
      else
        project_time_entries_url @project
      end
    end
end

# Don't pass in what can be derived from other assignments
class TimeEntry < ActiveRecord::Base
  before_save { self.spent_on ||= user.today }
end