How you would do depends on how the data is stored in the database table. If the start is stored as datetime - for example, '2015-12-15T21:00:00.00' then, and the number of working hours is stored as an int, you would simply add the number of hours to startdate, for example. DATEADD(hour, 8, StartDateTime)
I didn't understand what you meant by "both". If you can post sample data as it exists in your table that would help. Usually converting to character and doing the calculations on date/time is not a good approach.
It should be a simple query which most people who frequent on this forum should be able to write for you easily. But they can't do that because they don't know what your data structure is.
Is there one table, or more than one table involved? You said start time and end time are data types.
There is no untyped or dynamically typed data in SQL (if you set aside variants), so the fact that they are data types does not give any info.
Is there a staff table? with staff ID perhaps?
At the very least, provide some sample data from your tables and then type out some tabular data that you want to get based on that sample input data.
You need to post some sample data for people to be able to help you on this one. Create a simple CREATE TABLE statement where the columns mimic your real table and provide some insert statements to populate the table.