The Wolfram Documentation gives information on how to import an entire sheet, and an entire row, but not an entire column.
Currently, the code I have been trying to contort into a working form is:
voltage = Import["July 6th 2016.xlsx", "Data", 2, 3}]
This will import the 3rd row of the 2nd sheet. I cannot seem to figure out how to import a column instead though.