Date/Time format syntax

I'm trying to make my .NET program use .NET DateTime formatting syntax, which is robust, unambiguous and locale-aware.

I also need to be compatible with Excel, which continues to use its old-style syntax, even in Excel 2007, and it won't accept the newer .NET syntax.

For example, Excel uses "d/m/yyyy h:mm:ss am/pm" where .NET would use "d/M/yyyy h:mm:ss tt".

My question is simply: Why doesn't Excel use the better syntax?

Thanks,
Dennis Huber