It's excel limit.
Find ref. here:
http://office.microsoft.com/en-in/excel-help/excel-specifications-and-limits-HP010342495.aspx#BMcalculation
If your data are not use for calculation and only for display purpose, better you format it as TEXT.
Find ref. here:
http://office.microsoft.com/en-in/excel-help/excel-specifications-and-limits-HP010342495.aspx#BMcalculation
If your data are not use for calculation and only for display purpose, better you format it as TEXT.