1) You return zero as text, better as number. Even if that doesn't affect SUM, it affects formatting. Better to have all numbers in resulting column
2) Instead of DATEVALUE with date string better to use DATE or DATEVALUE with reference on another cell. If you open your file on PC with another locale or another default date settings in OS, you formula won't work
3) Not necessary to complicate the formula with DATEDIF to find difference in days. Dates in Excel are just integers, simple subtraction is enough.