Problems with date format

This is a two-part question....

1) I have a spreadsheet that includes information on single dates and multiple dates. I want to create a pivot table to make a report from some of the information. I want the report to be in chronilogical order. However, because of the the variation in date format, it is not completely chonilogical.

Here's an example of how the dates I originally listed in chronilogical order shift one I format the pivot table report:

4/16/07 - 4/18/07
4/20/07 - 4/22/07
4/21/07 - 4/22/07
4/16/2007
4/17/2007
4/21/2007

How can I write these dates so that it will work for chronilogical order?

This brings me to my next question...

2) When I type in a date, example: I type in EXACTLY 4/6/07, it automaticly changes the text to 4/6/2007. This is not because of how the cells are formatted (i.e. changing the way the date is displayed regardless of how I type it) but it actually changes the what I typed into the cell.

I went into Tools -> Options -> Error Checking tab -> unchecked the Rule 'Text date with 2 digit years' and it still didn't work. I also went into the Spelling tab and looked into the AutoCorrect options, and this date format wasn't one of them...

This has been frustrating me for much time now, so any help would be greatly appreciated!! Thank you.