The value passed into the format() function is coming from an XML file and has never been coerced.
Therefore, when checking is_int($value), the check always returns false - because it's a string.
Changing the check fixes the issue and Spout now correctly parses large numbers.
Although Excel has a Date type, older Excel versions use numeric values to store dates.
The value represents the number of days since Jan 1st, 1900.
The only way to tell if the value is a number or a date is to look at the styles.xml and check if the cell has date formatting.
Adrien Loison