Need to convert into PP (PowerPivot)

First, let's use Power Query to structure the data we need.

1. Format your source data as a table, and create a query which references this table
RawData:

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2. Now we can create a list of all possible start times, between the earliest and latest start times in your data, based on 30 min intervals:
TimeList:

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3. Close and Load both queries as 'Connection Only', and 'Load to Data Model'

4. In PowerPivot, create a relationship between RawData[Start Time] and TimeList[Start Time]

5. Add the following measure (you can add in either table,m I added it to TimeList, then hid RawData from client tools)

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6. Insert a pivot table, with Start Time and End Time as rows, and StaffCount as Values. Format to suit (tabular form, no subtotals, etc)

See attachment for worked example.