How can I calculate time with a large pool of raw data?

I_Want_To_Learn This  should be relatively easy with Power Query (PQ). Download and install the add-in as described in the link below if you really are on Excel 2013. In later versions, the functionality is incorporated in Excel. 

https://powerquery.microsoft.com/en-us/excel/ 

Once you have PQ, get familiar with this tool. It's amazingly powerful and not all that difficult to learn. You'll connect to the raw data, set correct data types (especially for dates and times) and append all the data from the individual sheets into one.

Then you can summarise (group, sort, filter etc.) the time records per employee, per day per account. You'll need to be clear, though, with the logic of calculating total duration of a task since the raw data contains only one time stamp. In other words, when does a task start and when does it end? For instance, what duration should be derived from row number 2 in your example? Perhaps you can upload a file with a more realistic example of the raw data you are dealing with. Replace any sensitive information though with some fake data, but leave the structure of the data as it is.