I just had the same problem - the number format button would not show up in the field list. So I reset the Pivot-data source to just the rows containing the data (I had previously just selected the whole columns ($A:$J)). With that done, the button reappeared and the date-format changes worked for the graph as well. Then, after formating the whole date coulumn to "Date" in the source data, it also worked with whole coulumns as the source, although that reformating step had not been successful before changing the Pivot-Data-Source. So there seems to be a bug which keeps a coulmn from being recognised as "date" as long as something else sits there.