develop a calculation tool via Excel

Hi, I have a simple project, I need a programmer to develop a tool via Excel. I am working with the recruitment department and I need to design a tool via Excel (for example) the aim of this tool is to calculate the total of work experience by years and months. Sometime the candidate comes and has worked for more than 15 companies in multiple periods, some of periods for a year and some for months. Sometimes we Facing difficulty to calculate manually to know the number of years of experience explicitly. I also want to design this tool in a way that cannot be modified. We need to have two accounts. One for a public user with generic password and the other one is administrator account. I also wish to pay attention of the tool in terms of performance and design.