Excel cells changes number format automatically
 

I am using Excel 2007. Many of the cells I am working on are in general
format where I store various numbers for analysis. These values change itself
to date format occassionally creating whole lot of work for me to change them
back to general format. How can I solve this issue?
Also in my pivot table, my source data is in general format. Still when I
click on sub total in any catagory in the table to find details about it,
formats are changed date format in the new sheet it pops up.
Can someone help resolve this issue.