I get the same problem with CAS numbers. Excel creates dangerous situations - it is not possible to have any vaguely "date-like, but not actually a date" data in Excel without it rapidly being corrupted if the file can be opened by users with multiple Excel
versions. It's bad enough with just one.
My latest challenge (Excel 2010):
Format a cell as text
Copy the text 31-08-9 into the cell from Word
It is instantly 31/08/2009 the cell now shows as format date.
Reformat the cell as text, and it now shows 40056
Type the text 31-12-7 into a cell formatted as text and immediately Excel warns you that you have a date with a two digit year. Excel is determined to find dates and Excel having once decided something is a date your data is no longer safe.
Conversely, type 31/12/1899 into a cell and ask Excel to display it in the form 13 March 2001 and Excel hasn't a clue what to do.
So not only is Excel dangerous in its appetite for making non date data into dates it's also pretty inept at handling actual dates once it gets them!
This is not Excel 0.9, this is supposed to be a mature software product.... Most Freeware authors would be embarrassed to be this bad!