That's a very good proposal. Current time sheets in Jira web interface are not very usable, so adding such functionality to Jira Client will make it a more valuable addition to Jira

. The basic idea of timesheet is to answer 2 questions:
1. "What tasks my employee worked on during last week/month?"
2. "How much time did he spend on this tasks?"
The first question is used to control progress during work. The second question is used to monitor time estimates and for billing information. As i can see, the best visual representation is 2 views. First one is focused on time and shows a list of tasks with time summary (even if a task was worked on multiple days it shows a single record and a summary time). Second one is focused on prgress and shows a days of months with list of tasks employee worked on during each day, alongside with time logged. The second view is optional for the first release, though.
Igor, I'm a developer myself, from Famatech/Moscow. If you are interested in feedback about GUI layouts and overall timesheet mechanics feel free to contact me via eye@famatech.com
I have a deep experience with task management, time tracking, Jira and Jira Client. And i will gladly help you with such great feature.