The time and date of the last modification of the attachment (optional). ISO 8601 format is used : yyyy - mm - ddThh : mm : ss, optionally followed by a timezone : Z, +hh : mm, or - hh : mm. The PDF reference is ambiguous; apparently, the modifiation time is supposed to be the time and date of the last modification of the file at the time it was attached.Changing the description does not update the modification date / time.

public string getModificationTime()