Commented Unassigned: EPPlus cannot perform date math on cells with dates that are quoted [15434]

If a cell has a date that is quoted and you call ExcelWorksheet.Calculate or ExcelRange.Calculate on a formula that performs math on that value, EPPlus throws a FormatException with the error "Input string was not in the correct format."

So for example, if cell C3 contains the formula ="12/31/2013" and you try to calculate the formula C3+1, EPPlus will throw a FormatException whereas Excel itself will recognize that the string value is a date and perform the math on it correctly (returning in this case the numeric equivalent of 1/1/2014).
Comments: Fixed in changeset 7bf69ce7cf44