Even if it opens Read only, you may be able to make changes in your chart,
but if you want to save those changes, you will probably have to save the
file with a different name or location.  
To change the way dates are displayed on the X-axis, right click on one of
the dates and choose, Format Axis.  In the Format Axis dialog box, click on
the Number option in the list to the left and click the date format you wish
to see.  Since you are making the dates longer, you may also want to check
out the Alignment option to make your dates easier to read.  Try changing the
text direction to rotate all text 90 degrees.
Good luck!