I have no Idea why the work diary was changed it use to be that you can view all hours worked at once.
Now they broke it up and it dont work right you click on it and cant click next rather you click back no idea on the flow really this did not need to be changed it worked great before hand and ive been using it for 6 years or so.
Could you please clarify what you are referring to? You can still see total hours worked that day in Work Diary on the top when viewing Work Diary:
You can also go to Reports> Timesheets and see total hours billed for a particular week.
You should be able to use back and next navigation for dates in the Work Diary: