You need to access the index of the data frame, not the columns. To do this you can do the following:
DF.index[x]
So in your case:
appl.index[0]
Incidentally if you want to get 'Date' out of the index, so you can reference it in the way you are accustomed to (i.e. as a column) you can reset the index:
appl.reset_index(inplace = True)
And then if you decide you don't like that you can just change it back:
appl.set_index('Date', inplace = True)