Negative number formats change when multiple users share a spreadsheet, causing frustration

[jkass]

My entire Finance Department wants to show negative numbers with parentheses -- as (1), not as -1. I created a custom format that does this for me, and which aligns the decimal points. When other people in my department open a file that I created, they see the negative number as I want them to see -- as (1) -- but the actual Excel format for that cell changes. My custom format is lost when they open the file. If they save the file, and then I open it, the number format is once again changed, to something undesirable, -1. How can I fix this problem? Here are the specifics:

I am using Windows 7 and Excel version 2010. I have this problem with my coworkers who are using exactly the same software. I also have this problem with my coworkers who are using Windows 8 and Excel version 2013. I think that the source of the problem is some setting on my PC, but I have not been able to find such a setting. All of us have set our Microsoft Office defaults set to show negative numbers with parentheses. It appears that Excel is overriding these defaults.

Specifics:
I start with a custom format of #,##0_);(#,##0) . The number is displayed as (1).
When a coworker opens the file, they see a format of (1,234) . The number is displayed as (1).
When I open the file after a coworker has saved it, I see a format of #,##0;-#,##0 . The number is displayed as -1.

One curiosity I have noticed is that I do not have a format of (1,234) on my PC. Instead, I have a format of -1,234.

Can you offer any suggestions?