Format continues to change to Date

[Jon Lester]

I currently have a File with 3 Data Pages ranging from 5K to 18K lines of Data. Off of these I have 8 pages of pivot tables and charts with 4-9 Charts per page.
Problem is that the file changes it' formats to Date. When I go through and change everything except Date columns to general the Pivot table Drop lists still are displaying a Date Format and I can’t change them making determining the selection a trial by error issue.
Why do I lose the format? Last month I completely pulled down all data and rebuilt the file because of this and it has resurfaced again.

 

[TCEMR]

Bless you! I have a large 200 sheet .xlsm workbook that is not shared. All of my General cells changed to Time format. I was in the middle of writing a bunch of code to manually change every cell on every sheet when you pointed me in the right direction. Not only did I need to change the "Normal" style to "General", but I also had to delete any custom cell formats (Format Cells--> Custom) that started with [$-409]. You have to unprotect all sheets first. Yesterday, the same problem happened again and I have no idea why but I was able to fix it again in 5 seconds. Thank you!