Excel adding days to specifc date gives error #VALUE!

Excel 2000 onward...
Content of B1 Cell : 17/12/2010
Content of A4 Cell : 7
Formula of C5 cell : =B1+A4

Gives error at C5 Cell #VALUE!

Solution:
With WinXp Change setting Date to dd/MM/yyyy ( Control Panel- Regional and Langauge Options - Customized - Date - Short Date )

After resolving problem content of C5 : 24/12/2010